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Gravimetric methods are expected to play a decisive role in geophysical modeling of the regional crustal structure applied to geoneutrino studies. GIGJ (GOCE Inversion for Geoneutrinos at JUNO) is a 3D numerical model constituted by ~46 x 10$^{3}$ voxels of 50 x 50 x 0.1 km, built by inverting gravimetric data over the 6{\deg} x 4{\deg} area centered at the Jiangmen Underground Neutrino Observatory (JUNO) experiment, currently under construction in the Guangdong Province (China). The a-priori modeling is based on the adoption of deep seismic sounding profiles, receiver functions, teleseismic P-wave velocity models and Moho depth maps, according to their own accuracy and spatial resolution. The inversion method allowed for integrating GOCE data with the a-priori information and regularization conditions through a Bayesian approach and a stochastic optimization. GIGJ fits the homogeneously distributed GOCE gravity data, characterized by high accuracy, with a ~1 mGal standard deviation of the residuals, compatible with the observation accuracy. Conversely to existing global models, GIGJ provides a site-specific subdivision of the crustal layers masses which uncertainties include estimation errors, associated to the gravimetric solution, and systematic uncertainties, related to the adoption of a fixed sedimentary layer. A consequence of this local rearrangement of the crustal layer thicknesses is a ~21% reduction and a ~24% increase of the middle and lower crust expected geoneutrino signal, respectively. Finally, the geophysical uncertainties of geoneutrino signals at JUNO produced by unitary uranium and thorium abundances distributed in the upper, middle and lower crust are reduced by 77%, 55% and 78%, respectively. The numerical model is available at http://www.fe.infn.it/u/radioactivity/GIGJ, Comment: 35 pages, 9 figures, 4 tables more...

The intercalation of [Cu(Phen)(H2O)2]2+ (CuPhen) in montmorillonite (Mt) produces a stable hybrid material that is very efficient in removing NH3 from gas phase even at extremely low pressures. The process was studied by elemental analysis, X-ray powder diffraction, thermal analysis coupled with evolved gas mass spectrometry and DR UV¿Vis, NMR and X-ray absorption spectroscopy. The adsorption of CuPhen on Mt consists of two consecutive steps. During the first one, CuPhen intercalates alone into Mt through a cation exchange process, afterwards CuPhen and SO42¿ ions entry jointly into the mineral interlayer. The two-steps adsorption process is described by a VI-type isotherm, successfully fitted by two independent Frumkin isotherms. NH3 trapping is long-lasting, easy, fast even at extremely low gas pressure and reversible under mild conditions. Mt containing CuPhen always results well performant in removing ammonia from gas phase, but an appreciably higher adsorption capacity of NH3 is obtained when SO42¿ ion is absent from the interlayer. This hybrid montmorillonite is thus a promising material to be used in industrial or environmental contexts, as an efficient air-cleaner., We acknowledge MIUR-PRIN2017 - PROGETTO N. 2017L83S77, the Spanish CRG BM25-SpLine, the Spanish Ministry of Science Innovation and Universities (MICIU), the Spanish National Research Council (CSIC) and The European Synchrotron (The ESRF) for the use of the synchrotron radiation facilities and financial support, the LADAC laboratory of DSCG, University of Modena and Reggio Emilia. A.S. acknowledges the financial support from the Comunidad de Madrid, Spain for an “Atracción de Talento Investigador” contract (No. 2017-t2/IND5395). more...

Rossiantonite, ideally Al3(PO4)(SO4)2(OH)2(H2O)10·4H2O, triclinic (space group P 1), a = 10.3410(5), b = 10.9600(5), c = 11.1446(5) A, α = 86.985(2), β = 65.727(2), γ = 75.064(2)°, V = 1110.5(1) A3, Z = 2, is a new mineral from the Akopan-Dal Cin cave system in the Chimanta massif (Guyana Shield, Venezuela). The mineral occurs as small (≤0.15 mm) and transparent crystals in a white to slightly pink fine-grained sand, filling spaces between boulders of weathered quartz sandstone. Associated phases are gypsum, sanjuanite, rare alunite, quartz and micro-spherules of amorphous silica. Rossiantonite is colorless with a white streak and vitreous luster. The mineral is brittle with irregular to sub-conchoidal fracture and it shows a poorly developed cleavage. Rossiantonite is biaxial and not pleochroic, with mean refractive index of 1.504. The calculated density is 1.958 g/cm3. Electron microprobe analyses, with H2O measured by thermogravimetric analysis, provided the following empirical formula based on 28 O apfu: Al2.96Fe0.03P1.01S2H30.02O28. The five strongest lines in the X-ray powder diffraction pattern, expressed as d (A), I , ( hkl ) are: 4.647, 100, (210); 9.12, 56, (100); 4.006, 53, (220); 8.02, 40, (110); 7.12, 33, (011). The crystal structure, refined using 3550 unique reflections to R = 0.0292, is built of PO4 and Al O6 polyhedral rings, creating complex chains parallel b by sharing the OH-OH edge belonging to the Al(3) polyhedron. Three symmetrically independent Al sites can be identified, namely: Al(1), Al(2), and Al(3). Tetrahedral sites, occupied by P, share all their apexes with AlO6 octahedra. Unshared octahedral apexes are occupied by water molecules. Four additional water molecules are placed in between the previously identified chains. Two oxygen tetrahedra, occupied by S atoms, are connected along the chains by means of weak hydrogen bonding. The rossiantonite structure shows similarities with minerals belonging to the sanjuanite-destinezite group. more...

Six Fe-bearing mica samples with different Fe ordering, Fe 2+ /(Fe 2+ +Fe 3+ ) ratio, octahedral, and tetrahedral composition were studied. Four micas belong to the phlogopite-annite join (space group C 2/ m ), two are Mg-rich annite and two are Fe-rich phlogopite, one is a tetra-ferriphlogopite (space group C 2/ m ) and one is Li-rich siderophyllite (space group C 2). Thus these samples had a different environment around the Fe cations and layer symmetry. These micas were characterized by chemical analyses, single-crystal X-ray diffraction, X-ray absorption spectroscopy, and magnetic measurements. In samples with Fe mostly in octahedral coordination, dominant magnetic interactions among Fe atoms are ferromagnetic, which reach a maximum at higher Fe 2+ /(Fe 2+ +Fe 3+ ) ratios. Samples with higher Fe 2+ /(Fe 2+ +Fe 3+ ) ratio are also characterized by higher values of the Curie-Weiss θ constant. Where Fe 2+ /(Fe 2+ +Fe 3+ ) ratios decrease, θ values also decrease. The Fe 3+ -rich phlogopite shows predominant Fe 3+ in tetrahedral coordination and shows anti-ferromagnetic interactions with a negative value of the Curie-Weiss θ constant (i.e., θ = −25 K). Fe ordering in octahedral trans - and in one of the two cis -sites accounts for a greater θ value in Li-rich siderophyllite when compared to other samples showing similar octahedral Fe content. Our data suggest that Fe 3+ cations and other non-ferromagnetic cations hinder long range magnetic ordering. This observation may produce for the different role of octahedral Fe magnetic interactions that can in principle develop along long Fe-rich octahedral chains, when compared to tetrahedral-octahedral interactions that are confined within the layer by the non-ferromagnetic cations of the interlayer. Spin glass behavior is indicated by the dependency of the temperature to produce maxima in the susceptibility curve. These maxima are related to the frequency of the applied AC magnetic field. more...

The crystal chemical features of the bulk and the uppermost (001) surface layers of freshly cleaved surfaces of two trioctahedral Fe-rich mica-1M (space group C2/m) polytypes, i.e. a tetraferriphlogopite from an alkaline-carbonatitic complex near Tapira, Belo Horizonte, Minas Gerais, Brazil, and an Fe2+-bearing phlogopite containing less tetrahedral Fe3+ from the Kovdor carbonatite-bearing, alkaline-ultrabasic complex, Kola Peninsula, Russia, are explored here. Mineral-surface effects were investigated by X-ray Photoelectron Spectroscopy (XPS) and compared to the bulk structure derived from single-crystal X-ray diffraction data. Based on microprobe analysis and the X-ray study, the chemical formulae are [XII](K0.99)[VI](Fe 0.08 2+ Fe 0.15 3+ Mg2.76Ti0.01)[IV](Fe 0.82 3+ Si3.18)O10.37F0.24(OH)1.39 and [XII](K0.94Na0.06)[VI](Fe 0.17 2+ Fe 0.05 3+ Mg2.75Mn0.01Ti0.05)[IV](Fe 0.16 3+ Al0.84Si3.00)O10.21F0.35(OH)1.44 for tetraferriphlogopite and Fe-bearing phlogopite, respectively. The tetrahedrally coordinated sites of the two minerals differ, where Fe-for-Si substitution is at 20.5% in tetra-ferriphlogopite and at 4% in Fe-bearing phlogopite. The bulk study showed that Fe3+ substitution increases the tetrahedral sheet thickness and the mean tetrahedral edge lengths in tetra-ferriphlogopite compared to Fe-bearing phlogopite. The tetrahedral rotation angle (α) changes remarkably from tetra-ferriphlogopite (α = 10.5°) to the Fe-bearing phlogopite (α = 8.5°), thus indicating a significantly greater initial lateral sheet misfit (leading to a greater tetrahedral ring distortion) between the tetrahedral and the octahedral sheets in the tetra-ferriphlogopite compared to Fe-bearing phlogopite. The Fe3+ substitution for Si and the differences in lateral dimensions of the tetrahedral and octahedral sheets affect the tetrahedral flattening angle (τ), with τ = 109.9° for tetraferriphlogopite and τ = 110.7° for Fe-bearing phlogopite. The binding energy (BE) of photoelectron peaks in XPS is dependent on the chemical state of atoms and on their local environment at the near surface. The Mg in both phlogopites is bonded to F, with the BE of Mg1s increasing as coordinated oxygen atoms are substituted by fluorine. For Fe-rich phlogopite (BE = 1306.8 eV), the binding energy is greater than for tetra-ferriphlogopite (BE = 1305.9 eV), and this is consistent with the bulk composition having greater F-for-OH substitution in Fe-rich phlogopite (F0.35vs. tetra-ferriphlogopite, F0.24 atoms per formula unit). more...

This study focuses on the crystal-chemical characterization of fluorannite from the Katugin Ta-Nb deposit, Chitinskaya Oblast’, Kalar Range, Transbaikalia, eastern Siberia, Russia. The chemical formula of this mineral is (K0.960Na0.020Ba0.001)(Fe2+2.102Fe3+0.425Cr3+0.002Mg0.039Li0.085Ti0.210Mn0.057)(Al0.674Si3.326) O10(F1.060OH0.028O0.912). This mica belongs to the 1M polytype (space group C2/m) with layer parameters a = 5.3454(2) Å, b = 9.2607(4) Å, c = 10.2040(5) Å, b = 100.169(3)º. Structure refinement, using anisotropic displacement parameters, converged at R = 0.0384. When compared to annite, fluorannite shows a smaller cell volume (Vfluorannite = 497.19 Å3; Vannite = 505.71 Å3), because of its smaller lateral dimensions and c parameter. Flattening in the plane of the tetrahedral basal oxygen atoms decreases with F content, together with the A–O4 distance (i.e. the distance between interlayer cation A and the octahedral anionic position) due to the reduced repulsion between the interlayer cation and the anion sited in O4. more...

The X-ray absorption angle-dependent behavior at the potassium K -edge has been determined for two end-members and two intermediate trioctahedral micas, which are representative of the Fe2+Mg−1, Fe3+Al−1, SiAl−1FeLi−1, and F−1OH exchange vectors, using horizontally polarized synchrotron radiation. Experimental spectra are interpreted by the multiple scattering theory following decomposition according to the angular dependence approach of Brouder (1990). The experimental spectra sum up anisotropic effects deriving from in-plane and out-of-plane photoelectron interactions with the potassium near- and next-near neighbors up to the fifth/sixth coordination sphere. For the first time, the absorption edge of a low Z atom is decomposed so as to produce two partial patterns giving, respectively, the full in-plane absorption spectrum (σ||) and the full out-of-plane spectrum (σ⊥). They fully describe the complete X-ray absorption dichroic behavior of layered compounds, of which mica is a prototype. σ|| essentially reflects the arrangement of the atoms located in the mica interlayer space and facing tetrahedral sheets. Differences among the four micas depend on structural changes induced by the adaptation of their tetrahedral sheets to the increasing sizes of their octahedral sheet due to chemical substitutions (i.e., the Fe2+Mg−1 vector, mainly) and to chemical changes at the octahedral anion site (F−1OH vector). By contrast, σ⊥ reflects multiple-scattering interactions entering deep into the mica structure, beyond the tetrahedral sheet and well into the octahedral sheet. Such contributions reflect both changes in the anions coordinating the octahedral cations and increasing total amount of heavy atoms (essentially Fe), and differ based on electronic properties such as oxidation states (e.g., Fe2+ and Fe3+). Therefore, angle-dependent XANES spectroscopy is one of the rare spectroscopic techniques able not only to detect, but also to quantify structural effects in the atoms at the medium- to long-range order. more...

Nb-containing titanite, [ 4 ] Si 1 . 0 0 0 [ 6 ] (Ti 0 . 7 4 8 Al 0 . 0 9 5 Zr 0 . 0 4 5 Nb 0 . 0 4 1 Ta 0 . 0 0 1 Fe 3 + 0.067 Sn 0 . 0 0 2 Sb 0 . 0 0 1 ) 1 . 0 0 0 [ 7 ] (Ca 0 . 9 8 6 Ce 0 . 0 0 4 La 0 . 0 0 1 Mn 0 . 0 0 4 Na 0 . 0 0 5 K 0 . 0 0 1 ) 1 . 0 0 1 F 0 . 0 5 5 (OH) 0 . 0 6 5 O 4 . 8 8 0 , from San Vito quarry, Mt. Somma, Naples, Italy, is characterised by extensive substitutions in the six- and seven-fold coordination sites. Single-crystal X-ray diffraction study indicates this niobian titanite to be monoclinic (a = 6.557(1), b = 8.701(1), c = 7.077(1) A, β = 113.86°) with C2/c symmetry. Crystal structure refinement (agreement factor, R = 0.027) suggests that the substitution mechanism (Al, Fe) 3 + + (Nb, Ta) 5 + = 2Ti 4 + is primarily responsible for the incorporation of pentavalent metal ions in the octahedral site. more...

For more than forty years, The Clay Minerals Society has dispensed a set of source clays which have enabled a large number of researchers to work on similar materials. Many of these source clays remained unchanged over the years but, conversely, other clays have gone out of stock and thus were replaced. This was the fate of montmorillonite STx-1a, which was replaced by STx-1b. Although STx-1a and STx-1b share many basic chemical and mineralogical features, some minor differences exist that can affect behavior. A baseline characterization of the source clay STx-1b, which was the objective of this study, was, therefore, necessary to provide researchers a tool useful not only for new investigation but also to compare new results obtained on STx-1b with literature data on STx-1a. This characterization was gained using traditional and advanced methods that included: 1) chemical composition (major and trace elements); 2) cation exchange capacity determination; 3) thermal analyses coupled with evolved gas mass spectrometry; 4) quantitative mineralogical characterization using powder X-ray diffraction and Rietveld-RIR (Reference Intensity Ratio) refinement; 5) X-ray absorption spectroscopy at the Fe K -edge; 6) diffuse reflectance ultraviolet-visible and infrared spectroscopies; and 7) 29 Si, 27 Al, and 1 H magic-angle spinning nuclear magnetic resonance measurements. According to this multi-analytical approach, the chemical formula for STx-1b is [4] (Si 7.753 Al 0.247 ) [6] (Al 3.281 Mg 0.558 Fe 0.136 Ti 0.024 Mn 0.002 ) [12] (Ca 0.341 Na 0.039 K 0.061 )O 20 (OH) 4 . more...

This paper, after an introduction about the state of the art in the methods for the evaluation of Hg chemical speciation, focuses on Hg polluted soils and sediments from Siele mining area (Piancastagnaio, Siena, Italy). In particular, the mineralogical composition was evaluated via X-ray powder diffraction, total Hg was determined via inductively coupled plasma optical emis- sion spectroscopy on acid digested samples, and Hg binding forms were investigated both via sequential selective extraction and pyrolytic extraction. Solutions obtained via sequential selective extraction were checked for Hg content using inductively coupled plasma optical emission spectroscopy, whereas to carry out pyrolytic extraction a thermogravimetric analyser coupled with an evolved gas mass spectrometer was used. To our knowledge, this last method was never used for this purpose on natural samples. Experimental results enhance that: i) Hg detection via thermal analyses can qualitatively distinguish among different Hg com- pounds. ii) The temperature of Hg thermal release is affected by both the sample matrix and the Hg bond type. iii) Thermal analyses fail in the attempt of a quantitative determination of the evolved species; this goal is better achieved via the selective chemical leach- ing. iv) Most of the Hg is related to HgS-like bonds, thus pointing to the presence of very stable Hg compounds. v) Only in some samples we found also organo-complexed Hg-like compounds and, in a limited amount, hydrosoluble Hg and/or Hg species soluble in weakly acid environment. vi) No correlation between sampling point and both total Hg and/or Hg speciation was observed. more...

This study describes the influence of temperature on two Hg(II)-cysteine modified clay minerals with different layer charges (i.e., montmorillonite and vermiculite) by using chemical and thermal analyses coupled with evolved gas mass spectrometry (TGA-MSEGA), synchrotron-based X-ray absorption spectroscopy (XAS), and X-ray powder diffraction (XRPD) techniques; these two latter techniques were applied on samples heated "in situ" up to 900 degrees C. Hg bonds cysteine by the thiol group with Hg-SH bond lengths similar in both clay minerals, thus suggesting their independence from layer charge. On the contrary, the thermal behaviour of the adsorbed organometallic complexes was strictly layer charge dependent, as evidenced by decomposition temperatures, which were lower in montmorillonite than in vermiculite. Results from X-ray absorption near edge structure (XANES) spectroscopy evidenced nearly complete Hg desorption in montmorillonite between 600 and 700 degrees C, unlike in vermiculite where Hg was still present at these values. (C) 2010 Published by Elsevier B.V. more...

abstra C t We recorded angle-dependent XANES (AXANES) spectra at the potassium K-edge for three compositionally intermediate polylithionite-siderophyllite trioctahedral 1 M-micas using polarized synchrotron radiation. We evaluated the experimental spectra for both their in-plane and out-of-plane component fractions of the electric dipole contribution using the analytical formulae of Brouder (1990), referring to theory to extract the origin of their multiple-scattering pathways of Natoli et al. (2003). This analysis was extended to a fourth lithian mica studied previously and allowed characterization of the local environment and ordering around the potassium atoms in the interlayer of the entire set of micas. The AXANES in-plane components are notably similar to the XANES spectra recorded on randomly oriented powders, provided these are oriented at the “magic angle” (Pettifer et al. 1992). Most observed contributions arise from multiple-scattering interactions of the photoelectron ejected from the potassium absorber colliding with atoms located in the interlayer itself. Note that this includes not only interactions with other coplanar potassium and/or alkali atoms distributed along the interlayer plane, but also with their near- and next-nearest neighboring oxygen atoms which lie on the basal planes of the tetrahedral sheets facing the interlayer. By contrast, the AXANES out-of-plane component suggests that several multiple-scattering pathways cross the energetic and structural barrier represented by the tetrahedral sheets. They reach not only the X anions that are located on the upper level of the octahedral sheets, at the center of the open cavity in the tetrahedral sheet, but also the metal cations centering the octahedral sheet itself. Therefore, the out-of-plane components provide indirect information on the number of independent octahedral sites, the cation oxidation state, and the trans- vs. cis-orientation of the anionic sites. more...

Microprobe analysis, single crystal X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray absorption spectroscopy were applied on Fe-rich osumilite from the volcanic massif of Mt. Arci, Sardinia, Italy. Osumilite belongs to the space group P6/mcc with unit cell parameters a = 10.1550(6), c = 14.306(1) A and chemical formula (K0.729)C (Na0.029)B (Si10.498 Al1.502)T1 (Al2.706 Fe 0.294 2+ )T2 (Mg0.735 Mn0.091 Fe 1.184 2+ )AO30. Structure refinement converged at R = 0.0201. Unit cell parameter a is related to octahedral edge length as well as to Fe2+ content, unlike the c parameter which does not seem to be affected by chemical composition. The determination of the amount of each element on the mineral surface, obtained through X-ray photoelectron spectroscopy high-resolution spectra in the region of the Si2p, Al2p, Mg1s and Fe2p core levels, suggests that Fe presents Fe2+ oxidation state and octahedral coordination. Two peaks at 103.1 and 100.6 eV can be related to Si4+ and Si1+ components, respectively, both in tetrahedral coordination. The binding energy of Al2p, at 74.5 eV, indicates that Al is mostly present in the distorted T2 site, whereas the Mg peak at 1,305.2 eV suggests that this cation is located at the octahedral site. X-ray absorption at the Fe L2,3-edges confirms that iron is present in the mineral structure, prevalently in the divalent state and at the A octahedral site. more...

Trioctahedral mica crystals are frequently found in the volcanoclastic products emplaced during the final stages of the activity of the Colli Albani volcano (Roman Region, central Italy). In the youngest phreatomagmatic deposits, mica is found either as a minor mineral phase in holocrystalline ejecta, scoriae, and coherent pyroclastics, or as loose phenocryst in incoherent pyroclastics. Based on optical and electron microscope investigations, as well as crystal chemical and structural data, the micas selected for this study were divided into two groups, the first one encompassing brownish, Fe-rich crystals (type-A phlogopites, hereafter referred as t-A) with 0.68 From a petrological point of view, the textural and chemical variations of t-A phlogopites are compatible with fractional crystallization processes taking place within the magma chamber. t-B phlogopites show evidence of a Ti-oxy substitution mechanism, thus suggesting high fO2 conditions. Textural and paragenetic features observed in t-B phlogopite-bearing rock samples, indicate a genesis by thermal metamorphism of a siliceous dolomitic limestone with the input of a variable amount of a potassic magma, possibly the same from which the t-A phlogopites formed, with the exception of one sample, for which a different parental magma is suggested. Indeed, there is an almost continuous spectrum of crystal-chemical and structural parameters starting from t-B and evolving towards t-A phlogopites. The t-A phlogopites displays complex, apparently meaningless relationships in their crystal-chemical parameters, thus indicating interplay of several substitution mechanisms. They possibly formed in polybaric conditions, since their cell volumes differ, but the total size of the cations hosted in the octahedral sheet is the same. more...

A crystal-chemical study of thirteen biotite (twelve of 1 M polytype and one of 2 M 1 polytype) and four muscovite samples was made. The biotite coexists with the muscovite. Samples are from metamorphic terranes and from granitic and granodioritic bodies occurring in three areas of western Maine. The metamorphic mineral zones identified by mineral compatibilities are, in order of increasing metamorphic grade: the Lower Sillimanite Zone (LSZ), the Upper Sillimanite Zone (USZ), and the K-feldspar + Sillimanite Zone (K + SZ). The muscovite compositions cluster near ideal muscovite and display a small celadonite substitution and a small, but variable, paragonite substitution. The biotite composition has values of VIMg2+/VI(Mg2+ + Fe2+) ranging from 0.26 to 0.54 and significant octahedral Al content (0.48 ≤ VIAl ≤ 0.72 apfu in metamorphic biotite samples, 0.51 ≤ VIAl ≤ 0.67 in those from granites). In trioctahedral micas from western Maine and especially in those with graphite, there are a greater number of interlayer vacancies than in common micas. Interlayer vacancies have an increase in interlayer cation-basal oxygen atom distances and a decrease in tetrahedral flattening angle τ, thus suggesting a reduced interlayer charge. With a few exceptions, tetrahedral rotation angle α is related to crystallization temperature. In particular, α decreases with a temperature increase, and α is also related to octahedral chemical substitutions. Results tentatively suggest, for micas from metamorphic environments, a direct influence of genetic parameters ( T and f O2) on mica crystal structure, and not just chemical composition. more...

Cell parameters and atomic coordinates for the true micas are varied to simulate layer deformation along the [001]* direction by an external force. The resulting (deformed) structures are then used to determine bonding forces and to calculate a maximum force component along the [001]*. Bonding forces are compared to experimental observations of bond lengths of the interlayer, octahedral, and tetrahedral sites. Calculated bonding forces are consistent with experimental observations that locate the cleavage plane along the interlayer. Because many studies have shown that the chemical composition of the cleavage surface often differs from the structure of the bulk, compositional variations were considered in determining cleavage energy. The chemical composition of the cleavage surface may produce a reduction in cleavage energy. This reduction in energy depends on various elements occurring in greater number at the cleavage surface than in the bulk. A reduction in cleavage energy occurs if there is a reduction in the interlayer site size, as measured by the area defined by the first-coordination basal oxygen atoms. In addition, a reduction in lateral cell dimensions and an increase in the bonding force between the basal oxygen atoms and the interlayer cation also results in a reduction in cleavage energy in the direction normal to the layer. Joins considered are phlogopite–annite, tetra-ferriphlogopite–tetra-ferri-annite, polylithionite–siderophyllite, muscovite–celadonite, and muscovite–paragonite. A lack of homogeneity in composition may produce preferential cleavage locations within the family of (001) planes. The cleavage energy appears to be greater for homogeneous synthetic micas compared to natural micas. more...

This work considers micas present both as phenocrysts in granular xenoliths and as large xenocrysts in the phreatomag-matic deposits, which characterize the final volcanic activity of Colli Albani volcanic complex, Roman Comagmatic Region (central Italy). In these micas the Fe/(Fe+Mg) ratio spans from 0.06 to 0.55, thus the micas range from phlogopite to magnesian annite through ferroan phlogopite. Heterovalent octahedral substitutions involve Ti, Al and also Cr but in very limited amounts. In the interlayer, Ba for K substitution occurs up to 0.06 apfu. Crystal structure refinements were determined in space groups C 2/m and C 2/c for the 1M and 2M 1 polytypes, respectively (agreement factor, R obs : 0.021 ≤ R obs ≤ 0.035). The crystal structure of these micas is greatly affected by the annitic substitution. An increase in the annitic component is reflected by a decrease in distortion affecting both individual tetrahedra (tetrahedral angle variance, TAV: 0.49 ≤ TAV ≤ 1.13° 2 ) and the entire tetrahedral ring [6.6 ≤ α ≤ 10.34°]. The interlayer topology is affected as well. A strong correlation exists between Fe and Ti; thus, the same correlations found for Fe apply to Ti also. Another strong correlation exists between heterovalent octahedral substitutions and the (OH)- site deprotonation. In contrast, heterovalent interlayer substitutions are accompanied by substitution of F- for (OH)-. Despite their scattered occurrence in pyroclastics, these micas nicely represent the overall crystal chemical trends of micas, which re-crystallized undisturbed in magma before this reached explosive conditions. One crystal of the dataset considered may represent a phenocryst in a hypabissal body disrupted by one of the many explosive phases of the Alban Hills volcanism. more...

The crystal chemistry of Ti-rich trioctahedral micas of plutonic origin, cropping out at Black Hill (South Australia) has been investigated by combining electron microprobe analysis, single crystal X-ray diffraction, Mossbauer spectroscopy and X-ray photoelectron spectroscopy. Chemical analyses have shown the samples taken to be quite homogeneous and Ti-rich (TiO2 ≈ 7 wt.%). Mossbauer investigation yielded Fe2+/Fe3+ ≈ 30. X-ray photoelectron spectroscopy analysis seems to suggest the occurrence of three Ti species: octahedral Tr4+(60%), octahedral Ti3+(26%), and tetrahedral Ti4+(14%). The analyzed sample belongs to the 1M polytype and the relevant crystal data from structure analysis are: a = 5.347(1) A, b = 9.261(2) A, c = 10.195(2) A, β = 100.29°(1). Anisotropic structure refinement was performed in space group C2/m, and converged at R = 2.62, Rw = 2.80. Structural details (the c cell parameter, the off-center shift of the M2 cation towards 04, the bond-length distortions of the cis-M2 octahedron, the interlayer sheet thickness, the projection of K−O4 distance along c*, the difference outer- inner) support the occurrence of the Ti-oxy substitution (VIR2+ + 2(OH)− ⇌ Ti4+ + 202− + H2) in the sample. Analysis of structural distortions as a function of the Ti content revealed that the positions of the oxygens 03 and 04 are displaced in opposite senses along [100]. This produces an enlargement of the M1 site with respect to the M2 site and a shortening of the interlayer distance. This trend seems to be in common with other Ti-rich 1M micas of plutonic origin. more...

The structure of dioctahedral true micas such as muscovite and celadonitic muscovite (2M1 polytype, space group C2/c) is mostly affected by variations of the octahedral Al (VIAl) content. Crystals with greater Mg, Fe substitutions (i.e. celadonitic muscovite) reduce the dimensional difference between the larger trans-oriented M1 site and smaller cis-oriented M2 octahedral site. The octahedral anionic position O4 is displaced from the center of the hexagon, defined by 031 and 032 oxygen atoms (i.e. ‘octahedral hexagon’), both on and off the (001) plane. The distance between interlayer cation A and O4 is smaller in more substituted species, thus providing different orientations of the O4−H vector, as a function of VIAl. Octahedral distances ( and are expressed as a function of cell parameters and VIA1 content, thus allowing an approximate estimate of site dimensions. These approximations are useful when a detailed structural refinement is not available. In celadonitic muscovite, the octahedral hexagon mean edge ( Hex) is not significantly affected by VIA1 content. The VIA1 increase produces both a decrease in cell lateral dimensions and a distorted ‘octahedral hexagon’. The decrease in a and b is consistent with a decrease of Hex, whereas the distortion of the’ octahedral hexagon’ is consistent with an increase of ( Hex), because an irregular hexagon produces a longer mean edge than a regular hexagon of equal area. The tetrahedral mean basal edge (VI bassal) is reduced as celadonitic substitution progresses. The tetrahedral rotation angle, α was thus found to increase from celadonite to muscovite. However, in muscovite with VIAl content between 1.8 and 2.0 atoms per formula unit (a.p.f.u.), α approaches a saturation value, thus showing a proportional increase of tetrahedral and octahedral sheet lateral dimensions. Furthermore, α variation allows a coarse approximation of the threshold VIAl content, below which celadonitic substitution may not progress. more...

Synchrotron-based c tended X-ray absorption fine structure spectroscopy (EXAFS). X-ray diffraction at room and high temperature, thermal analyses combined with mass spectrometry of evolved a-as. and chemical analyses contributed to assess the influence of mercury on montmorillonite and vermiculite layers. The adsorbed Hg amount was higher for montmorillonite (Hg=37.7 meq/100 2) than for vermiculite (He-28.0 meq/100 g). The basal spacing for the Hg treated samples was 15.2 (montmorillonite) and 14.5 Angstrom (vermiculite). Thermal and evolved gas spectrometry analyses suggest that Hg was released at T congruent to 230 degreesC and at 600 degreesC for montmorillonite. but at 550. 800 and 860 C for vermiculite. The effect of temperature on Hg release is also apparent when the basal spacing at 230 degreesC for montmorillonite (d(001)=10.3 Angstrom) is compared to that for vermiculite (d(001)=11.8 Angstrom). EXAFS analyses provide qualitative evidence that oxygen atoms occupy the first coordination shell of Hg in both clay minerals. The best fit between observed and calculated spectra is obtained when montroydite is assumed as a reference model compound. more...

The crystal chemistry of ten hydroxyl fluorapatite crystals from the Tapira alkaline carbonatitic complex (Brazil) was investigated by electron microprobe, ion microprobe, infrared spectroscopy and single crystal X-ray diffraction. Apatite crystals are characterized by modest REE and a very low Cl content. They exhibit the following compositional range in atoms per formula unit 9.935 less than or equal to Ca less than or equal to 10.024, 5.926 less than or equal to P less than or equal to 6.006, 0.005 less than or equal to REE less than or equal to 0.046, 0.422 less than or equal to F less than or equal to 0.914, 0.003 less than or equal to Cl less than or equal to 0.008, 0.003 less than or equal to Si less than or equal to 0.038; Na is present in trace. Crystal structure refinements were carried out in space group P6(3)/M, (R values varies between 0.016 and 0.028). The REE for Ca substitution requires two different exchange mechanisms producing an increase in the bond valence on the X anionic site: the deprotonation of OH and/or the substitution of CO32- for OH. The CO32- substitution in the anionic sites of investigated structures was related to great CO2 activity of the melt whereas deprotonation of the apatite crystals could be related to the highly oxidizing nature of the Tapira carbonatitic magma. The very low Cl/F ratio suggests that the crystallization of Tapira pluton was relatively shallow. The determination of REE partitioning between the Ca1 and Ca2 sites is difficult because of the low REE content. A proportional increase in the size of the Ca1 and Ca2 sites was observed with increasing REE content as well as an overall increase of a parameter. more...

The aim of this work is an introductory study on the crystal chemistry of igneous muscovite, with particular focus on samples from the Pedrobernardo pluton (central Spain), whose crystal structure and chemistry is compared with data from the literature. This pluton is a layered sheet-like granite body with distinct horizontal zoning, which can be subdivided into three main zones (lower, middle, and upper) characterized by different crystallization conditions. Different chemical trends can be identified when the chemical compositions of muscovite from the Pedrobernardo pluton are compared with literature data. Samples from the middle and lower zones follow a similar trend, whereas samples from the upper zone follow a different trend. In samples from the middle and lower zones, Ti 4+ increases without affecting Fe content (Fe 2+ and/or Fe 3+ , referred to here as Fe). Fe and Mg 2+ can enter into the layer in similar amounts. In samples from the upper zone, the Ti 4+ content is lower than in the lower and middle zones, whereas Fe is higher. Fe content is related to Ti 4+ content and seems to substitute for octahedral Al 3+ more easily than Mg 2+ . Samples from the middle and lower zones are characterized by substitutions involving both tetrahedral and octahedral sites, whereas samples from the upper zone are characterized by a preference for octahedral substitutions. Single-crystal structure refinements suggest a partial occupancy of the octahedral trans -site (Ml) related to substitution of F-for OH-, and probably also to a different O-H vector orientation. Lateral cell parameters are found to be affected by the octahedral cis -site mean bond distance (〈;M2-O〉), which is linked to Fe 2+ content for a large number of the samples under consideration. The 〈;M1-O〉 distance as well as tetrahedral thickness are found to be affected by the c parameter, following the structural mechanisms induced by chemical substitutions described in previous studies. These parameters thus appear to be affected more by electrostatic interactions involving the whole layer rather than by site occupancy and local chemical composition. more...

Single-crystal X-ray diffraction experiments were carried out on roscoelite crystals from Reppia, Val Graveglia, Italy. Roscoelite [structural formula: XII(Ba0.006K0.994)IV(Si3.150Al0.850) VI(Al0.040Fe0.150Mg0.100Mn0.062V1.696Ti0.003)O10(OH)2] shows a near-perfect three-dimensional stacking order with cell parameters a = 5.292(1), b = 9.131(2), c = 10.206(3) A, β = 100.98(2)° and space group C2/m, which indicate a 1M polytype. The crystal structure was refined on the basis of Fo2 for 846 unique reflections to R1 = 3.29% calculated using 746 unique observed reflections [|Fo| ⩾ 4σ(Fo)]. The mean tetrahedral cation–oxygen atom distance, = 1.641 A, is close to the mean value obtained for dioctahedral true micas from the literature, whereas the octahedral sheet is characterized by a larger cis-octahedral cation–oxygen atom bond distance = 2.020 A which, together with the mean electron count, is consistent with V occupancy. The presence of V within the octahedral sheet produces the smallest tetrahedral rotation (α = 2.3°), the lowest flattening of the basal oxygen surface (Δz = 0.118 A) and the narrowest interlayer separation (3.030 A) in dioctahedral micas. more...

In order to evaluate the possible use of natural fluor-hydroxyapatite (FOHAp) crystals as soil additive for the in situ remediation of heavy-metal contaminated soils, the crystal structure of FOHAp crystals, used in batchexperiments at 22 °C with pH within the range 4.8-9.5 and in presence of Zn 2 + ions, was determined. Crystal structures (space group P6 3 /m) converged to R = 0.026 for untreated FOHAp and to R = 0.028 (Experiment 1) and R = 0.032 (Experiment 2) for Zn 2 + -treated FOHAp crystals. The site occupancy ratios [(Zn-Ca)2/(Zn-Ca)1] obtained from the refinement of electron density is = 1.500 for natural FOHAp, 1.500 (Experiment 1) and 1.520 (Experiment 2) in Zn 2 + -treated crystals. The value deriving from experiment 2 agrees with small changes in Ca2 site occupancy. Ca2O 6 X polyhedron distortions may be related both to a slight preference of Zn for Ca2 and to OH for F substitution during the overall reaction sequence. more...

Natural and synthetic micas have been used widely as substrates to study biological systems; but, as in the case of negatively charged DNA, anionic charge repulsion may render micas a less than ideal templating surface for many biological systems. The purpose of this study was to investigate the potential for the chlorite clinoclore, which contains a positively charged interlayer octahedral sheet, to serve as a substrate for DNA adsorption. The relationships between clinochlore cleavage characteristics, in terms of nano-morphology, and surface potential are investigated, as are its average crystal chemistry and topology. That the structural features of clinochlore can be used successfully to condense, order, and self assemble complex biomolecules, such as DNA is also proven. A natural IIb-4 clinochlore [ $C\bar 1$ symmetry, unit-cell parameters a = 0.53301(4); b = 0.92511(6); c = 1.4348(1) (nm); α = 90.420(3); β = 97.509(3); γ = 89.996(4) (°)] with chemical composition $\left( {{\rm{M}}{{\rm{g}}_{1.701}}{\rm{Fe}}_{0.245}^{2 + }{\rm{T}}{{\rm{i}}_{0.004}}{\rm{A}}{{\rm{l}}_{0.998}}{\rm{Cr}}_{0.052}^{3 + }} \right)\;{\rm{M}}{{\rm{g}}_3}\;\left( {{\rm{S}}{{\rm{i}}_{2.939}}\;{\rm{A}}{{\rm{l}}_{1.015}}\;{\rm{Fe}}_{0.046}^{3 + }} \right){{\rm{O}}_{10}}\left( {{\rm{O}}{{\rm{H}}_{7.913}}{{\rm{F}}_{0.087}}} \right)$ was selected. The octahedral sites of the silicate layer ( = 0.2080 nm and = 0.2081 nm) are equal and occupied by Mg, whereas the octahedral sites in the interlayer M(3) and M(4) ( = 0.2088 nm and = 0.1939 nm) show different sizes and are mostly completely occupied by divalent (Mg2+ and Fe2+) and trivalent (Al3+) cations, respectively. The clinochlore cleaved surface is present in two forms: (1) the stripe type (0.40 nm in height, up to several micrometers long and ranging from some nanometers to a few microns in lateral size); and (2) the triangular type (0.40 nm in height). Both features may result either from interlayer sheets whose cleavage weak directions are related to the different M(3) and M(4) site occupancy, or from weak interlayer bonding along specific directions to the 2:1 layer underneath. The cleaved surface, particularly at the cleaved edges, presents high DNA affinity, which is directly related to an average positive surface and ledge potential. more...

Experimental and theoretical EXAFS and XANES data on Fe coordination and bond distances have been obtained for a natural tetra-ferriphlogopite. XANES data show a predominance of trivalent iron in tetrahedral coordination. The use of theoretical multiple scattering calculations helps distinguish the contributions to the total spectrum coming from the different iron atoms. EXAFS spectra were reduced and analyzed using the GNXAS programs and Fe has been confirmed to be in tetrahedral coordination, with refined structural data Fe-O = 1.86 A, Fe-Si = 3.24 A, Fe-O-Si = 138°. Moreover, a further contribution from Fe is detected corresponding to a Fe-O distance equal to 2.22 A. This latter contribution (20 mol % of the total Fe ca. ) is interpreted to arise from divalent Fe in octahedral coordination, in excellent agreement with the structural and chemical data. more...

Rectorite crystals [[4](Si6.81Al1.19)[6](Al3.26Ti0.04Fe0.553+Mg0.18Mn0.01)[12](Na0.02K0.88 Mg0.16Ca0.01)O20(OH)4·0.75H2O] found in micaschist of the Sesia-Lanzo Zone (NW Italy) were studied using a variety of techniques including microprobe analysis, infrared spectroscopy, single crystal and powder X-ray diffraction, atomic force microscopy and thermal analysis. Chemical data and exchangeable cation determination indicate that K+ is the dominant non-exchangeable interlayer cation, and thus is believed to occupy the mica interlayer site; Mg2+ together with small amounts of Ca2+, Na+ and K+ represent the exchangeable cations and can therefore be related to the smectite-like component. The coefficient of variation, CV, of d(00l) values (CVnatural = 0.47; CVglycolated = 0.43) demonstrates the regularity of the mica-smectite interstratification, whereas the unit-cell parameters obtained by single crystal methods suggest different layer-stacking models. more...

To verify chromium enrichment of the muscovite layer, a crystal chemical and XANES study on chromium-containing muscovite crystals from the South Island of New Zealand was carried out. The crystals studied differ from those of end-member muscovite in that they display variable levels of octahedral substitutions and homovalent and heterovalent substitution of K in interlayer sites. Single-crystal X-ray diffraction data were collected for three crystals in the space group C 2/ c to an agreement factor (R obs ) from 0.025 to 0.033. Tetrahedral cation disorder was found for each sample and the values of mean bond length for both tetrahedra do not depart significantly from that of the end-member muscovite-2 M 1 . Electron density at the M2 site is greater than that required for the ideal muscovite-2 M 1 structure, and a small excess of electron density is found for two crystals in M1. As the octahedral substitution of larger cations for Al increases in the octahedral sites, the match between tetrahedral and octahedral sheets improves and tetrahedral rotation angle, α, decreases. XANES spectra at the Cr K-edge in these chromium-containing muscovite samples exhibit octahedral symmetry. Moreover, a careful analysis of the pre-edge region shows at least two features. A qualitative fitting procedure of the pre-edge region indicates that no more than 0.5% of total Cr(III), if any, may occupy the tetrahedral site. more...

The crystal structure and M-site populations of a series of micas-1 M from miarolitic pegmatites that formed within host granitic rocks of the Precambrian, anorogenic Pikes Peak batholith, central Colorado, were determined by single-crystal X-ray diffraction data. Crystals fall in the polylithionite-siderophyllite-annite field, being 0 ≤ Li ≤ 2.82, 0.90 ≤ Fe total ≤ 5.00, 0.26 ≤ [6] Al ≤ 2.23 apfu. Ordering of trivalent cations (mainly Al 3+ ) is revealed in a cis-octahedral site (M2 or M3), which leads to a lowering of the layer symmetry from C 12 /m (1) (siderophyllite and annite crystals) to C 12(1) diperiodic group (lithian siderophyllite and ferroan polylithionite crystals). On the basis of mean bond length, the ordering scheme of octahedral cations is mostly meso-octahedral, whereas the mean electron count at each M site suggests both meso- and hetero-octahedral ordering, the calculated mean atomic numbers being M1 = M3 ≠ M2, M2 = M3 ≠ M1 and M1 ≠ M2 ≠ M3. As the siderophyllite content increases, so do the a , b, and c unit-cell parameters, as well as the refractive indices, primarily n β . The tetrahedral rotation angle, α, is generally small (1.51 ≤ α ≤ 5.04°) and roughly increases with polylithionite content, whereas the basal oxygen out-of-plane tilting, Δz, is sensitive both to octahedral composition and degree of order (0.0 ≤ Δz ≤ 0.009 A for siderophyllite and annite, 0.058 ≤ Δz ≤0.144 A for lithian siderophyllite and ferroan polylithionite crystals). more...

A comparison was made between single-crystal structure refinements, electron microprobe analyses and octahedral site populations of seven biotite crystals and data obtained previously for coexisting muscovite in peraluminous granites using the same methods. Both micas, from several plutons of Northern Victoria Land (Antarctica) and Sardinia (Italy), show significant octahedral substitutions: biotite has a composition characterized by relatively high [6] Al content (0.32 ≤ [6] Al ≤ 0.59 apfu), whereas muscovite is characterized by phengite-like substitutions [0.12 ≤ [6] (Mg + Fe + Ti + Mn) ≤ 0.35]. Mean bond-lengths and electron count data for six biotites-1 M (space group C 2/ m, agreement factor 2.7% ≤ R obs ≤ 3.6%) and a biotite-2 M 1 (space group C 2/ c , R obs = 2.8%) show that Al substitutes for divalent cations in the octahedral M2 site and that the Fe and Mg distribution is disordered. The mean tetrahedral bond lengths determined for biotite-2 M 1 reflect Al-Si disorder. In coexisting muscovite-2 M 1 crystals, small positive electron density residuals close to M1 site position as well as the increase in M2 mean atomic number is in agreement with the presence of a significant phengitic component. Reduction in biotite unit-cell dimensions with the increase of Al follows a pattern similar to that of associated muscovites, and the octahedral site volumes of both micas are influenced by the Al saturation index (ASI) of the rock. These results, and the calculated partition coefficients between biotite and muscovite for elements in M sites, are consistent with continuous reaction and re-equilibration of biotite and muscovite during crystallization of peraluminous granitic melts. more...

This work provides a crystal-chemical description of trioctahedral micas from volcanic rocks (lavas, tuffs, ignimbrites, and xenoliths) outcropping at Mt. Sassetto (Tolfa district, Tuscan Province, central Italy). Mica crystals vary in composition from ferroan phlogopite to magnesian annite. Heterovalent octahedral substitutions are mainly related to Al3+, Ti4+, and, only in a few samples, to Fe3+. The two main mechanisms regulating Ti inlet into the mica structure are the Ti-oxy and Ti-vacancy substitutions. In these micas, Ti content is the predominant crystal-chemical parameter and significantly affects octahedral and interlayer topology as well. Micas wiith the highest Ti contents deviate from the expected fractional crystallization trend in the Ti vs. Mg/(Mg + Fetot) diagram, possibly as a consequence of a variation in intensive parameters (T, P, fH2, fO2, fH2O) during crystallization in the magmatic chamber. In micas with significant Fe3+ contents, the layer contents show mean electron counts greater for the octahedral M2 site than for M1, and unit-cell parameters significantly smaller than usual for other micas. Atmospheric weathering is unlikely to produce the observed Fe oxidation. The Fe3+-vacancy and Fe-oxy substitutions could represent secondary, re-equilibration effect related to post-eruptive water-rock hydrothermal processes (acid-sulfate alteration). more...

This study investigates the complexes formed between amino acids, which are the natural degradation products of organic matter, and smectites. Thus, the adsorption and desorption behavior of cysteine and Na-, Ca-, Cu-homoionic smectites with different layer-charge location, a montmorillonite, and a beidellite, were studied. The clay samples were treated with Na, Ca, and Cu 1 N solutions and then with a 0.2 M cysteine solution. To test smectite-cysteine stability at acidic pH, the solids obtained were repeatedly treated with distilled water acidified to pH = 5. All treated samples were characterized by thermal, X-ray diffraction, chemical, and infrared analyses. The results showed that: 1) Na- and Ca-rich smectites adsorbed and retained small amounts of cysteine, and did not show interlayer cation-cysteine complexes, whereas the amino acid was strongly retained in the interlayer by Cu-rich smectites; 2) d(001)-values for Na- and Ca-rich smectites showed little or no expansion, whereas for the Cu-rich smectites the intercalation of the organic molecule produced a swelling of the structure; 3) the interaction mechanism of homoionic smectites with cysteine in an aqueous medium occurs by weak interactions, (e.g., van der Waals interactions, hydrogen bonding, dipole-dipole interactions, and other electrostatic forces such as entropy-driven hydrophobic bonding), and/or by complexes involving interlayer cations and organic ligands. The formation of a stable chelate complex with the saturating ion permits cysteine to be adsorbed by Cu(II)-rich smectites and to be resistant to migration in soils and groundwaters. more...

Phengitic muscovite-2M 1 crystals [ (super [12]) (K (sub 0.88-0.99) Na (sub 0.01-0.09) Ca (sub 0.00-0.06) Ba (sub 0.00-0.01) ) (super [6]) (Al (sub 1.64-1.88) Fe (super 2+) (sub 0.06-0.29) Fe (super 3+) (sub 0.01-0.16) Mg (sub 0.00-0.16) Mn (sub 0.00-0.07) Ti (sub 0.00-0.06) ) (super [4]) (Si (sub 2.87-3.30) Al (sub 0.70-1.13) )(OH) (sub 1.56-2.07) F (sub 0.00-0.41) O (sub 9.91-10.25) ] from pegmatites and peraluminous granites were refined to investigate the influence of phengitic substitution on the mica structure. Single-crystal X-ray diffraction data were collected for eleven crystals in the C2/c space-group (agreement factor 2.1% < or = R obs , < or =3.9%). Tetrahedral Si and Al cation disorder was found for each sample, with the mean tetrahedral cation-oxygen distances ranging from 1.639 Aa < or ={T1-O}< or =1.647 Aa and 1.640 Aa < or ={T2-O}< or =1.646 Aa. As phengitic substitution increases, the octahedral sheet expands and requires a less distorted (more hexagonal) tetrahedral ring (7.70 degrees < or =alpha < or =11.38 degrees ) and low corrugation of the basal O plane (0.1796 Aa< or =Delta Z< or =0.2296 Aa). The electron density at the M2 site is greater than that required for the ideal muscovite-2M 1 structure, and a small excess of electron density is found in the M1 site. The inner sixfold coordination of the interlayer (A) cation is elongated along c (super *) , which is consistent with the high alpha values and the long A-O11 bond length. more...

This paper explores the crystal chemical features of the bulk and the outermost (001) surface layers of two trioctahedral Li-rich mica-1 M (space group C 2) polytypes, i.e., a polylithionite (MLG-114) from Li-mica granitic pegmatite at St. Austell (SW England) and a Fe 2+ -rich polylithionite (Ch-140) from a rhyolite at Profitis Ilias, Chios Island, Greece. Structural formulas are [xii] (K 0.952 Na 0.019 Rb 0.019 ) [vi] (Al 1.034 Li 1.459 Fe 0. 2+ 389 Fe 0. 3+ 046 Mn 0.038 Mg 0.002 Zn 0.002 Ti 0.001 ) [iv] (Al 0. 3+ 477 Si 3.523 )O 10.081 (F 1.735 OH 0.184 ) and [xii] (K 0.992 Na 0.014 ) [vi] (Al 0.980 Li 1.028 Fe 0. 2+ 787 Fe 0. 3+ 022 Mn 0.059 Mg 0.052 Zn 0.010 Ti 0.024 ) [iv] (Al 0. 3+ 857 Si 3.143 ) O 10.095 (F 1.617 OH 0.288 ) for MLG-114 and Ch-140, respectively. Each mineral is characterized by a high F content in the anion site and has tetrahedral and octahedral compositions related to the exchange vector [vi] Li −1 [iv] Si −1 [vi] Fe 2+ [iv] Al. Unit-cell dimensions are a = 5.251(1), b = 9.066(2), c = 10.087(2) A; β = 100.694(5)° for polylithionite MLG-114 and a = 5.282(1), b = 9.121(3), c = 10.080(3) A; β = 100.764(5)° for Ch-140. Crystal structure refinements (agreement factors are R = 3.58% and 3.75% for MLG-114 and Ch-140, respectively) demonstrate that the [vi] Li −1 [vi] Fe 2+ [iv] Si −1 [iv] Al exchange vector produces a decrease in the lateral dimensions of the tetrahedral and octahedral sheets. The decrease in basal oxygen distances results from the effect of the strain caused by the orientation of opposing tetrahedral sheets within a 2:1 layer. The decrease reduces the strain so that the basal oxygen plane can remain nearly planar. Changes in these dimensions via distortions of the tetrahedral basal-oxygen ring (α = 3.3° and 4.1° for MLG-114 and Ch-140, respectively) are limited. Octahedral M1 and M3 sites are similar in size and much larger than M2 and the mean electron count is M3 Element concentrations on the (001) surface, obtained through X-ray photoelectron spectroscopy (XPS) high-resolution spectra for Si 2p , Al 2p , Fe 2p , K 2p , Li 1s , and F 1s core levels, indicate that a greater amount of lithium and a smaller amount of potassium characterize the surface with respect to the bulk. The decrease in K content, commonly observed in micas, is related to its location on the cleavage surface, because the cation must be distributed equally between the two (001) surfaces generated upon cleavage. The increase in Li content on or near the (001) cleavage surface suggests a preference for cleavage near lithium-enriched regions. The surface structure of the polylithionite crystals suggests that Al, Li, and Fe cations maintain coordination features at the surface similar to the bulk. Silicon, however, which is generally in fourfold coordination, shows also a small number of [1]-fold coordinated components at a binding energy of 99.85 eV. more...

Well-crystallized euhedral crystals of aluminian lizardite-2H 2 (Mg (sub 2.35) Fe (super 2+) (sub 0.06) Fe (super 3+) (sub 0.07) Al (sub 0.52) ) (Si (sub 1.41) Al (sub 0.59) )O (sub 5.00) (OH) (sub 4.00) were found near Schio (Vicenza, Italy). To gain insight into the role of a high Al content lizardite, chemical analyses and single-crystal X-ray data collection were conducted. Structure refinement, completed in space group P6 3 (agreement factor R = 0.034), gives mean T-O values of 1.654 Aa and 1.664 Aa for T1 and T2 sites, respectively. The ditrigonal distortion of the six-membered tetrahedral ring is positive (alpha = +9.7 degrees ), as expected for the 2H 2 polytype. The octahedral site has a mean bond length similar to that of the Mg-rich octahedra of amesite and distortion parameters similar to those of Al-rich octahedra. more...

A Ba-rich celestine (Sr0.87Ba0.13SO4) filling cavities of volcanoclastic rocks from Montecchio Maggiore (Vicenza, Italy) was studied. The role of the Ba content in the mineral was determined using X-ray powder data, single crystal X-ray refinement, thermal and chemical analyses. The unit cell parameters (obtained by single crystal diffraction) are a = 8.408, b = 5.372, c = 6.897 Å, and the refinement in the space group Pnma (Z = 4) gives the final R value of 0.039. The average (Sr,Ba)-oxygen bond length is 2.842 Å and agrees with an occupancy of Sr 87% and Ba 13%. Individual bond lengths (Sr,Ba)-O and bond strength calculations confirm that all twelve interactions are significant and define an irregular array around the cation. more...

Two smectites with different layer charge localization were used to characterize the chemical and structural aspects of minerals treated with 1.9 × 10−2m and 1 m heavy metal solutions (Zn2+ and Pb2+). Natural and treated smectites were analysed by: (1) X-ray powder diffraction first at 25°C and relative humidity of 60% and then at increasing temperature up to 400°C; (2) thermo-gravimetric and differential thermal analysis; (3) X-ray fluorescence; (4) atomic absorption and UV visible spectrophotometries. The sorption of both metals depends on 2:1 layer features and is enhanced when the total layer charge is mostly due to substitution in the octahedral sheet. The variability between Zn2+- and Pb2+-treated smectites in the interlayer water content depends both upon cation electrostatic features and upon smectite layer charge localization. The greatest water content was observed in [Zn2+]1 m-treated smectite showing charge imbalance in the octahedral sheet. In smectites treated with [Zn2+]1 m solutions, an additional thermal reaction occurs at a temperature of ∼380°C, and the d(001) spacing, which is 10 Å at a temperature of ∼200°C, gradually decreases in the temperature range 200–400°C. In smectites treated with 1 m [Pb2+] solutions this value is reached at a temperature of ∼150°C and remains quite constant up to 400°C. more...

This contribution is finalized at the discussion of the magnetic structure of two samples, belonging to phlogopite–annite [sample TK, chemical composition IV(Si2.76Al1.24) VI(Al0.64Mg0.72 $$ {\text{Fe}}_{1.45}^{2 + } $$ Mn0.03Ti0.15) (K0.96Na0.05) O10.67 (OH)1.31 Cl0.02] and polylithionite–siderophyllite joints [sample PPB, chemical composition IV(Si3.14Al0.86)VI(Al0.75Mg0.01 $$ {\text{Fe}}_{1.03}^{2 + } $$ $$ {\text{Fe}}_{1.03}^{3 + } $$ Mn0.01Ti0.01Li1.09) (K0.99Na0.01) O10.00 (OH)0.65F1.35]. Samples differ for Fe ordering in octahedral sites, Fe2+/(Fe2+ + Fe3+) ratio, octahedral composition, defining a different environment around Fe cations, and layer symmetry. Spin-glass behavior was detected for both samples, as evidenced by the dependency of the temperature giving the peak in the susceptibility curve from the frequency of the applied alternating current magnetic field. The crystal chemical features are associated to the different temperature at which the maximum in magnetic susceptibility is observed: 6 K in TK, where Fe is disordered in all octahedral sites, and 8 K in PPB sample, showing a smaller and more regular coordination polyhedron for Fe, which is ordered in the trans-site and in one of the two cis-sites. more...

Trioctahedral micas from kamafugitic lavas of Presidente Olegario (Brazil) are Ti-Fe-bearing fluorophlogopites. They were investigated using a combination of electron probe microanalysis (EPMA), single-crystal X-ray diffraction (SCXRD), and Mossbauer spectroscopy. EPMA data yielded the following ranges: Al2O3 (8.7–10.1 wt%), MgO (19.6–20.6 wt%), FeO (5.7–6.5 wt%), TiO2 (5.9–7.5 wt%), K2O (8.9–9.7 wt%), Na2O (0.4 –0.6 wt%), and fluorine (3.4–4.1 wt%). Mossbauer investigation indicated: VIFe2+ ~ 60%, VIFe3+ ~ 10%, IVFe3+ ~ 30%. X-ray analysis indicated the 1 M polytype, with cell parameters in the range a = 5.3208–5.3376, b = 9.2210–9.2464, c = 10.1227–10.155 A, β = 100.157–100.194°. Structure refinements using anisotropic displacement parameters were performed in space group C 2/ m and converged to 2.01 ≤ R 1 ≤ 3.51, 2.00 ≤ wR 2 ≤ 3.51%. The micas have no VIAl3+ and Na is probably partitioned over both interlayer and octahedral sites. Major substitutions are OH− ↔ F−, and the Ti-oxy substitution: VIM2+ + 2(OH)− ↔ VITi4+ + 2O2− + H2↑. Accordingly, the structural features of the studied micas encompass those typical of both fluorophlogopites and micas affected by Ti-oxy substitution. more...

A multi-methodical approach has been applied for characterizing the bulk and surface crystal chemical features of a high-temperature vesuvianite crystal from skarns of Mount Somma-Vesuvius Volcano (Naples, Italy). Vesuvianite belongs to the space group P4/nnc with unit cell parameters a = 15.633(1) A, c = 11.834(1) A and chemical formula (Ca18.858 Na0.028 Ba0.004 K0.006 Sr0.005 □0.098)19.000 (Al8.813 Ti0.037 Mg2.954 Mn0.008 Fe 0.114 2+ Fe 1.375 3+ Cr0.008 B0.202)13.511 Si18.000(O0.261 F0.940 OH7.799)9.000. Structure refinement, which converges at R = 0.0328, demonstrates a strong positional disorder down the fourfold axes, indicating that the Y1 site is split into two positions (Y1A and Y1B) alternatively occupied. However, because of X4 proximity to Y1B and Y1A, X4 cannot be occupied if Y1B or Y1A are. Overall Y1 occupancy (Y1A + Y1B) reaches approximately 0.5, as common in vesuvianite and occupancy of Y1B site is extremely limited. Moreover, T1 position, limitedly occupied, accommodates the excess of cations generally related to Y position. A small quantity (0.202 apfu) of boron is sited at the T2 site that, like T1, is poorly occupied. The determination of the amount of each element on the (100) vesuvianite surface, obtained through X-ray photoelectron spectroscopy high-resolution spectra in the region of the Si2p, Al2p, Mg1s, and Ca2p core levels, evidences that a greater amount of aluminum and a smaller amount of calcium characterize the surface with respect to the bulk. Although both of these features require further investigation, we may consider the Al increase can be related to preferential orientation of Al-rich sites on the (100) plane. Furthermore, the surface structure of vesuvianite suggests that Al, Ca, and Mg cations maintain coordination features at the surface similar to the bulk. Silica, however, while presenting fourfold coordination, shows also a [1]-fold small coordinated component at binding energy 99.85 eV, due to broken Si–O bonds at the surface. The presence of eight- and nine-coordinated Ca cations is suggested by a large XPS feature resulting from the merging of Ca2p3/2 and Ca2p1/2 peaks at 348.45 and 352.05 eV, respectively. more...

We investigated the crystal-chemical features of six crystals of lizardite-1 T sampled in four outcrops of ophiolite, at Pompeano, Sassomorello, Varana, and Santa Scolastica, in the Modena Apennines, Italy. In spite of the extensive contributions already present in the literature, this is the first study dealing with lizardite from the Modena ophiolites. As is clear from one of our samples, the whole-rock composition affects the composition of lizardite, which makes it susceptible to overprinting by later metasomatic events. In our study, we paid particular attention to the effects of VI Mg −1 VI Fe 2+ and of IV Si −1 IV Al VI (Mg, Mn, Fe) 2+ −1 VI (Al, Cr, Fe) 3+ exchange mechanisms on the structure. Our results suggest that VI Mg −1 VI Fe 2+ substitution induces an increase in the M –O4 length in octahedra and a decrease in the octahedral-site distortion. Both these effects are also observed to influence the unit-cell parameter c . The effect of the IV Si −1 IV Al VI (Mg, Mn, Fe) 2+ −1 VI (Al, Cr, Fe) 3+ substitution is to decrease the M –O1 distance and concomitantly, to increase the T –O1 distance. more...

Pseudobrookite-like minerals from two outcrops of Spanish lamproites (Jumilla, Murcia province, and Cancarix, Albacete province) cover the compositional range: Fe 1.6 3+ Mg 0.2 Ti 1.2 O 5 −Fe 0.6 3+ Mg 0.7 Ti 1.7 O 5 −Fe 0.2 3+ Fe 0.6 2+ Mg 0.3 Ti 1.9 O 5 thus they range between pseudobrookite and armalcolite. Al, Cr, Mn, Ca, Zr and Nb are minor elements. Single crystal X-ray structural refinements, in the space group Bbmm, give R-values between 0.016 and 0.024. The structural refinements show that: (1) M1 site has wider variations in mean bond lengths and is larger and more distorted than M2 site because of the preferential ordering in the M1 site of Mg and Fe 3+ in respect to Ti; (2) the cation ordering is probably controlled both by chemical environment and temperature of crystallization. In particular, ordering in pseudobrookites from rocks of comparable composition increases under decreasing temperature of crystallization; (3) in Jumilla rocks, the highest in oxygen activity among the lamproites analyzed (above the NNO buffer), all the iron in pseudobrookite occurs in the trivalent state, whereas in Cancarix and Las Minas de Helin, close to NNO and QFM buffers respectively, iron is present also in the divalent state more...

Biotites from mafic rocks occurring at different stratigraphic levels of the Ivrea-Verbano Mafic Complex are studied. The rocks are gabbros and diorites. All the biotites are intermediate between phlogopite and annite [0.28 more...

Ten single-crystal X-ray structure refinements of igneous and metamorphic Ba-bearing micas were determined to define the exchange vectors required to compensate for the excess in positive layer charge caused by [XII] Ba 2+ for [XII] K + substitution. The determined exchange vector is BaAlK −1 Si −1 . The micas have a Ba 2+ content in the range 0.01-1.09 apfu (atoms per formula unit) with octahedra mostly populated by Mg 2+ (2.21 ≤ Mg 2+ ≤ 3.07 apfu) and Fe 2+ (0.13 ≤ Fe 2+ ≤ 1.53 apfu). All samples are 1M polytype and the structure were refined in space group C2/m (0.020 ≤ R ≤ 0.034). lncreasing the substitution of [IV] Al 3+ for [IV] Si 4+ distorts the tetrahedra by progressively increasing the dimensions of the basal edges. This increase in the lateral dimensions of the tetrahedral sheet allows for a greater rotation angle α (5.9 ≤ α ≤ 11.5). Octahedral sheet crystal chemical features (ordering pattern, size and distortion of both M(1) and M(2) sites) are similar to those reported for micas in the phlogopite-annite join. The most notable variation involves the length of M(1) unshared edges, which decreases in samples with the highest [XII] Ba 2+ content more...

Kogure and co-workers have published several papers reporting the stacking structures of various 2:1 layer phyllosilicates ( e.g. aspidolite, Kogure et al. (2004); wonesite, Kogure et al. (2005); pyrophyllite, Kogure et al. (2006)), mainly by using high-resolution transmission electron microscopy (HR-TEM). However, there is ambiguity in wording that refers to the lateral displacement between two tetrahedral sheets across the interlayer regions. Zvyagin et al. (1969) used the symbol “τ” for this displacement, which describes the structures of pyrophyllite and talc, but no universally accepted wording was used for other 2:1 phyllosilicates or 1:1 phyllosilicates. Bailey (1984, p. 32) referred to the “layer offset” for this displacement in mica, but the original meaning of ‘layer offset’ can be taken as the ‘displacement’ to offset the intralayer shift of the 2:1 layer. A … more...

Factors controlling the crystal structure of phlogopite have been widely investigated; but the role of electrostatic interactions, for example, has received much less attention than other factors. The purpose of the present study was to peform a single-crystal refinement of an Al-saturated phlogopite and to use that refinement to supplement crystal-chemical analyses. The phlogopite investigated was from the Rumford quadrangle, Maine, and has the following chemistry: ${\left( {{{\rm{K}}_{0.81}}{\rm{N}}{{\rm{a}}_{0.03}}} \right)_{\Sigma \; = \;0.84}}\;{\left( {{\rm{M}}{{\rm{g}}_{1.84}}{\rm{Fe}}_{0.52}^{2 + }{\rm{A}}{{\rm{l}}_{0.45}}{\rm{M}}{{\rm{n}}_{0.02}}{\rm{Ti}}_{0.07}^{4 + }} \right)_{\Sigma \; = \;2.90}}{\left( {{\rm{S}}{{\rm{i}}_{2.78}}{\rm{A}}{{\rm{l}}_{1.22}}} \right)_{\Sigma \; = \;4}}{{\rm{O}}_{10}}{\left( {{\rm{O}}{{\rm{H}}_{1.81}},{{\rm{F}}_{0.05}}} \right)_{\Sigma = 1.86}}$ . The sample is a 1M polytype with C2/m symmetry and cell dimensions of a = 5.3220(4), b = 9.2170(7), c = 10.2511(8) A, and β = 100.081(1)°. Hydrogen atoms were located and the crystal structure was refined to give parameters R1 = 0.0301 and weighted R2 = 0.0887. The octahedral M1 site was larger than the M2 (average M1—O: 2.079 A, average M2—O: 2.062 A) and the electron counts were equal (M1 = M2 = 14.8 e−); based on bond distances, which are more accurate than electron counts in determining occupancy; this result is consistent with a slight preference of Mg for M2 and Fe2+ for M1. Thirty-five Al-rich, natural phlogopite-1M samples that are of (1) high metamorphic grade, and that have (2) total Al contents ⩾ 1.27 atoms per formula unit (a.p.f.u.), (3) Fe3+ contents ⩽ 0.11 a.p.f.u., and (4) Mn contents ⩽0.10 a.p.f.u. along with the newly described phlogopite, exhibited crystal chemical trends related to increasing Al content. Octahedral substitutions of smaller, high-charge cations (i.e. Al) apparently decrease distortions in the octahedral sites and produce longer M2—O4 distances. In addition, VIFe-F avoidance apparently occurs in high Al-content samples, which are generally high in VIFe. The data set also shows that these samples have limited ordering among M sites (Fe2+ in M1 and Al in M2), an increase in β (99.96° to 100.32°) possibly caused by cation ordering and therefore size differences of M1 and M2, and interlayer (A) sites with A—Oouter distances that increase and A—Oinner distances that decrease with increasing Ti content. Computer models were used to simulate electrostatic interactions in phlogopite structures with variable Al concentrations utilizing Pauling’s electrostatic valency principle, which considers first-coordination electrostatic interactions. The model results were compared to the maximum Al concentrations in natural and synthetic phlogopite samples. Model results revealed no indications (e.g. a limit reached or a sudden change occurred) that charge saturation/undersaturation of the apical oxygen atoms at Al contents equal to the maximum in natural and/or synthetic samples causes instability that could not be balanced by bond-length variation. However, a cation of higher charge substituting at M1 (or M2) may result in higher electrostatic repulsions between the other octahedral sites. Thus, the Al3+ content in the octahedral sites may reach a maximum, with Fe2+ for Mg substitutions favored. more...