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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...

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...

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 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...

Knowledge of the atomic arrangement of the topmost surface of micas is basic for the understanding of a large number of surface phenomena and interacting processes on the Earth's crust. Crystal structure of micas at the surface shows relaxation and reconstruction phenomena such as variation of coordination number for the interlayer cation and compositional variation from the composition of the bulk. These evidences support the hypothesis that small interlayer cations cluster at the cleavage plane and that a lack of homogeneity of the layers constituting the mica structure may produce preferential cleavage locations. 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...

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...

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...

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...

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...

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...

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...

This work relates the crystal chemistry and the magnetic behaviour of a trioctahedral mica (chemical formula: (K0.90Na0.01Ca0.01Ba0.01 ☐0.07)(Al0.05Fe2+1.10Mg1.38Ti0.32Mn0.01☐0.04)(Al1.12Si2.88)O10 (F0.27OH1.27O0.46); unit cell parameters: a = 5.345(2) Å, b = 9.261(4) Å, c = 20.189(8) Å; β = 95.075(8)°) from Minto Block (Ungava peninsula, northern Quebec, Canada), a region characterized by high magnetic anomalies. Crystallographic and X-ray absorption spectroscopy data suggest a prevalent divalent oxidation state for Fe and a disordered Fe 2+ distribution in the two octahedral sites Ml and M2. The real part of magnetic susceptibility shows two peaks at ∼5.2 K and 120 K. However, as demonstrated by AC magnetic susceptibility measurements, the origin of the two effects is different: the peak position of the first one (i.e. the effect revealed at 5.2 K) is frequency-dependent, thus suggesting a spin-glass like behaviour. The effect at 120 K can instead be attributed to the occurrence of diluted phases in mica matrix, such as Fe oxides. more...

The crystal chemistry of three Li-, Fe-, and Mn-rich trioctahedral micas has been characterized by single-crystal X-ray diffraction. The samples are from Hirukawa mine, Japan: (Si3.43Al0.57)(Al1.00 Fe0.38Mg0.01Mn0.17Li1.44)(Na0.05K0.95)O10F1.88(OH)0.12; from Mokrusha mine, Russia: (Si3.30Al0.70)(Al1.00Fe0.36 Mg0.01Mn0.31Li1.32)(Ca0.01Na0.04K0.94)O10F1.91(OH)0.09; and from Sawtooth Mountains, Boise County, Idaho, U.S.A.: (Si3.11Al0.89)(Al0.91Ti0.02Fe0.46Mg0.03Mn0.52Li1.06)(Na0.05K0.92Rb0.02)O10F1.89(OH)0.11. Our crystals belong to the 1M polytype with layer symmetry C121(1) and show M1 and M3 sites much larger in size than M2. Mean electron-count (m.e.c.) values are more variable for the M1 and M3 sites than for M2. With the exception of the sample from Sawtooth Mountains, all tetrahedral mean bond lengths appear to be smaller for T1 than for T11 site. When compared to the Li- and Fe-rich series, crystals show similar crystal-chemical trends, thus suggesting that the layer structure is affected in a similar way by Fe and Mn cations. more...

Publisher Summary This chapter describes structures and mineralogy of clay minerals. Phyllosilicates considered in this chapter ideally contain a continuous tetrahedral sheet. Each tetrahedron consists of a cation, T, coordinated to four oxygen atoms and linked to adjacent tetrahedra by sharing three corners (the basal oxygen atoms, Ob) to form an infinite two-dimensional hexagonal mesh pattern along the a, b crystallographic directions. The free corners (the tetrahedral apical oxygen atoms, Oa) of all tetrahedra point to the same side of the sheet and connect the tetrahedral and octahedral sheets to form a common plane with octahedral anionic position Ooct. Ooct anions lie near to the center of each tetrahedral 6-fold ring, but are not shared with tetrahedra. The 1:1 layer structure consists of the repetition of one tetrahedral and one octahedral sheet, while in the 2:1 layer structure one octahedral sheet is sandwiched between two tetrahedral sheets. In the 1:1 layer structure, the unit cell includes six octahedral sites (i.e., four cis and two trans-oriented octahedral) and four tetrahedral sites. Six octahedral sites and eight tetrahedral sites characterize the 2:1 layer unit cell. Structures with all the six octahedral sites occupied are known as “trioctahedral.” If only four of the six octahedra are occupied, the structure is referred to as “dioctahedral.” The structural formula is often reported based on the half unit-cell content—that is, it is based on three octahedral sites. more...

The crystal chemistry of three Li-bearing mica-2M(2) crystals from pegmatites has been studied by chemical analyses and single crystal X-ray diffraction; their belonging to the trilithionite-polylithionite join is highlighted by the following compositional ranges in atoms per formula unit [based on O12-(x+y) (OH)(x)F-y] : 3.198 more...

Chemical analyses and crystal structure refinements were performed on lithian siderophyllite-3 T crystals from granitic pegmatites of the anorogenic Pikes Peak batholith (Colorado) to characterize the crystal chemistry and relations with trioctahedral lithium-bearing micas showing different stacking sequences. Chemical data show that the studied samples fall on the siderophyllite-polylithionite join, closer to the siderophyllite end-member. Single-crystal X-ray refinements were carried out on three samples (two of which were taken from core and rim of the same crystal) in space-group P 3 1 12 (the agreement factor, R obs , varies between 0.034 and 0.036). Mean bond distances and mean electron counts of M1, M2 and M3 octahedral sites indicate an ordered cation distribution with M1 and M3 positions substantially larger than M2. In the sample with the largest iron content, the M2 mean electron count increases as well as the mean distance, whereas remains smaller than or . The tetrahedral cation-oxygen atom mean distances range from 1.614 to 1.638 A and from 1.663 to 1.678 A for T1 and T2 sites, respectively, being consistent with Al 3+ enrichment in the T2 sites. The tetrahedral rotation angle, α, is generally small (3.1 ≤ α ≤ 4.6°) and decreases with siderophyllite content. As Fe increases, the T1 tetrahedron becomes flatter (112.4 ≤ τ T1 ≤ 110.5°), whereas T2 tetrahedron distortion appears unchanged (110.7 ≤ τ T2 ≤ 110.9). more...

This work provides a crystal-chemical description of the trioctahedral sheet in 1 M mica polytypes of C 2/ m symmetry. Some octahedral parameters were found to be related strictly to octahedral chemical composition, whereas others were related to the overall layer chemistry. As a general rule, the M1 site is more affected by octahedral chemical composition than the M2 site, and octahedral distortions are more affected than octahedral size. This behavior is attributed to the constraints related to edge-sharing octahedra and octahedral-tetrahedral coupling, which involves the sharing of the apical tetrahedral oxygen atom between the tetrahedron and octahedron. A more quantitative explanation was sought by considering expressions that relate octahedral parameters as a function of atomic coordinates and unit-cell parameters. This procedure identified those parameters affected by local distortions and those that are a function of a more global effect and whose variation greatly affects the layer. Local distortions are obviously more likely a result of local chemical composition. The size and distortion of the octahedral sheet seem to be independent for the database of samples considered. The α angle was found to be a complex parameter required to: (1) fit tetrahedral and octahedral lateral dimensions; (2) fit tetrahedral topology to its local chemical composition, and (3) fit the interlayer cation electrostatic interactions. more...

The purpose of this report is to describe the appropriate use of indices relating to crystallinity, such as the ‘crystallinity index’, the ‘Hinckley index’, the ‘Kubler index’, and the ‘Arkai index’. A ‘crystalline’ solid is defined as a solid consisting of atoms, ions or molecules packed together in a periodic arrangement. A ‘crystallinity index’ is purported to be a measure of crystallinity, although there is uncertainty about what this means (see below). This report discusses briefly the nature of order, disorder and crystallinity in phyllo-silicates and discusses why the use of a ‘crystallinity index’ should be avoided. If possible, it is suggested that indices be referred to using the name of the author who originally described the parameter, e.g. ‘Hinckley index’ or ‘Kubler index’, or in honor of a researcher who investigated the importance of the parameter extensively, e.g. ‘Arkai index’. In contrast to a crystalline solid, an ‘amorphous’ solid is one in which the constituent components are arranged randomly. However, many variations occur between the two extremes of crystalline vs. amorphous. For example, one type of amorphous material might consist simply of atoms showing no order and no periodicity. Alternatively, another amorphous material may consist of atoms arranged, for example, as groups of tetrahedra ( i.e. limited order) with each group displaced or rotated ( e.g. without periodicity) relative to another. Thus, this latter material is nearly entirely amorphous, but differs from the first. Likewise, disturbance of order and periodicity may occur in crystalline materials. The terms ‘order’ and ‘disorder’ refer to the collective nature or degree of such disturbances. Although seemingly simple notions, ‘crystalline’ and ‘amorphous’ are complex concepts. Crystalline substances may show a periodic internal structure based on direction. For example, two-dimensional periodicity is common in phyllosilicates where two adjacent sheets or layers must mesh. For example, in serpentine, … more...