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Ferrofettelite, ideally [Ag6As2S7][Ag10FeAs2S8], is a new mineral (IMA2021-094) from the Glasberg quarry, Nieder-Beerbach, Odenwald, south-western Germany. It occurs as anhedral to subhedral flakes and grains up to 80 μm, associated with proustite and xanthoconite, on arsenolite, calcite and prehnite. Ferrofettelite is opaque with a metallic lustre and possesses a dark reddish-grey streak. It is brittle with an uneven fracture; the Vickers microhardness (VHN20) is 122 kg/mm2 (range 111–131). The calculated density is 5.74 g/cm3 (on the basis of the empirical formula). In plane-polarised reflected light, ferrofettelite is greyish white. Between crossed polars it is weakly anisotropic with red internal reflections. Electron-microprobe analyses give the chemical formula Ag16.04(Fe0.55Hg0.40Cu0.02)Σ0.97(As3.94Sb0.03)Σ3.97S15.02 on the basis of total atoms = 36.Ferrofettelite is monoclinic, space group C2, with a = 26.011(2), b = 15.048(1), c = 15.513(1) Å, β = 90.40(1)° and V = 6071.9(7) Å3 for Z = 8. The six strongest Bragg peaks in the powder X-ray diffraction pattern (d, Å (I, %) (hkl) are: 3.18 (50) ($\bar{8}$01), 3.104 (100) (005), 3.004 (60) ($\bar{8}$02), 2.755 (40) ($\bar{4}$43), 2.501 (30) ($\bar{4}$44) and 1.880 (30) ($\overline {12}$40). The crystal structure can be described as the alternation of two kinds of layers along the c-axis: layer A with general composition [Ag6As2S7]2– and layer B with a general composition of [Ag10FeAs2S8]2+. In the structure, the Ag atoms adopt various coordinations extending from quasi linear to quasi tetrahedral, the AsS3 groups form pyramids as are typically observed in sulfosalts, and mixed (Fe,Hg) links two sulfur atoms in a linear coordination. Ferrofettelite is the first reported inorganic phase showing a linear coordination for Fe2+. The high-temperature behaviour of ferrofettelite was studied up to 410 K and compared to that of fettelite.

The crystal structure of a new high-pressure hydrous phase, Si-rich Mg-sursassite, of ideal composition Mg4Al5Si7O23(OH)5, that was produced by sub-solidus reaction at 24 GPa and 1400 °C in an experiment using a model sedimentary bulk composition, has been determined by single-crystal X-ray diffraction. The phase was found to be topologically identical to Mg-sursassite, Mg5Al5Si6O21(OH)7, and has space group P21/m and lattice parameters a = 8.4222(7), b = 5.5812(3), c = 9.4055(9) Å, b = 106.793(8)°, V = 423.26(6) Å3, and Z = 1. The empirical formula determined by electron microprobe analysis of the same crystal as was used in the X-ray experiment is [Mg3.93(3)Fe0.03(1)]Σ3.96[Al4.98(3)Cr0.04(1)]S5.02 Si7.02(4)O23(OH)5, with hydroxyl content implied by the crystal-structure analysis. The most significant aspect of the structure of Si-rich Mg-sursassite is the presence of octahedrally coordinated Si. Its structural formula is M1,VIIMg2M2,VIMg22+M3,VI(Al0.5Si0.5)2M4,VIAl2M5,VIAl2T1,IVSi2T2,IVSi2T3,IVSi2 O23(OH)5. Si-rich Mg-sursassite joins the group of hydrous ultrahigh-pressure phases with octahedrally coordinated Si that have been discovered by experiment, and that may play a significant role in the distribution and hosting of water in the deep mantle at subduction zones. The reactions defining the stability of Si-rich Mg-sursassite are unknown, but are likely to be fundamentally different from those of Mg-sursassite, and involve other ultrahigh-pressure dense structures such as phase D, rather than phase A.

The crystal structure and chemical composition of a crystal of Mg-bearing phase Egg with a general formula M1−x3+Mx2+SiO4H1+x (M3+ = Al, Cr; M2+ = Mg, Fe), where x = 0.35, produced by subsolidus reaction at 24 GPa and 1400 °C of components of subducted oceanic slabs (peridotite, basalt, and sediment), was analyzed by electron microprobe and single-crystal X-ray diffraction. Neglecting the enlarged unit cell and the consequent expansion of the coordination polyhedra (as expected for Mg substitution for Al), the compound was found to be topologically identical to phase Egg, AlSiO3OH, space group P21/n, with lattice parameters a = 7.2681(8), b = 4.3723(5), c = 7.1229(7) Å, β = 99.123(8)°, V = 223.49(4) Å3, and Z = 4. Bond-valence considerations lead to hypothesize the presence of hydroxyl groups only, thereby excluding the presence of the molecular water that would be present in the hypothetical end-member MgSiO3·H2O. We thus demonstrate that phase Egg, considered as one of the main players in the water cycle of the mantle, can incorporate large amounts of Mg in its structure and that there exists a solid solution with a new hypothetical MgSiH2O4 end-member, according to the substitution Al3+ ↔ Mg2+ + H+. The new hypothetical MgSiH2O4 end-member would be a polymorph of phase H, a leading candidate for delivering significant water into the deepest part of the lower mantle.

We report laser ablation inductively coupled plasma mass spectrometry measurements of the trace element contents of the two naturally occurring quasicrystalline minerals, Al63Cu24Fe13 icosahedrite and Al71Ni24Fe5 decagonite, from their type locality in the Khatyrka meteorite. The isolated quasicrystal fragments were mounted separately from any matrix and are larger than the laser beam diameter. When the elements are sorted in order of volatility, a systematic and unique pattern emerges in both bulk natural quasicrystal specimens. They are highly depleted compared to primitive solar system materials (chondritic meteorites) in moderately refractory elements (those with 50% condensation temperatures near 1350-1300 K; V, Co, Mg, Cr) and significantly enriched in moderately volatile elements (those with 50% condensation temperatures between 1250 and 500 K; Sb, B, Ag, Sn, Bi). We compare the chondrite-normalized trace element patterns and ratios of the quasicrystals to those of scoriaceous cosmic spherules and other meteoritic components. The nonmonotonic shapes of the chondrite-normalized trace element patterns in both icosahedrite and decagonite are incompatible with a single condensation process from the gas of the solar nebula. Previous transmission electron microscopy studies show that the natural quasicrystals contain 3-5 vol % of silicate and oxide nanoparticle inclusions, which we consider to be the main host of the measured trace elements. On this basis, we construct a three-stage model for the formation of the quasicrystals and their inclusions: a high-temperature condensation stage and a low-temperature vapor-fractionation stage to make nanoparticles, followed by a third stage that leads to the formation of quasicrystals incorporating the two different types of nanoparticles and their incorporation into the CV chondrite parent body of the Khatyrka meteorite.

Most geobarometers use chemical compositions of minerals and their host melt to estimate crystallization pressures. Crystal structural parameters such as cell and site volumes are not usually considered despite their known sensitivity to pressure. Here, we compare two clinopyroxene geobarometers based upon electron microprobe analysis alone and coupled with single-crystal X-ray diffraction data. The case study is the plumbing system of Popocatepetl volcano (Mexico), which consists of three distinct magma reservoirs in upper, middle and lower crustal depths, represented by three compositionally and texturally distinct clinopyroxene populations (T1, Mg# core 82, and low-Ca with a lower wollastonite component). These clinopyroxenes are augites of limited compositional variability, although yielding a significant increase in cell (V cell) and M1 site (V M1) volumes from low-Ca and T2 core clinopyroxenes to T1 (core and rim) and T2 (rim) clinopyroxenes. This variation is not due to chemical or temperature effects but is linked to their depth of crystallization. The application of the geobarometer based on chemical composition alone is unable to distinguish the three different reservoirs postulated on volcanological and petrological grounds. In contrast, the application of the geobarometer based on both structural parameters and chemical composition yields a remarkable correlation between the calculated cell volume and the estimated depth of crystallization of the different clinopyroxenes, including core to rim differences. These results have twofold implications. First, the determination of the structural parameters of clinopyroxenes is the only method to resolve the actual distribution of Mg, Fe2+, Fe3+ in the M1 and M2 structural sites and, given the sensitivity of cell and site volumes to pressure, permits to improve geobarometric estimates in volcanic plumbing systems. Second, the quantitative determination of the crystallization depth of the different clinopyroxenes has permitted to rescale the depth of the three different reservoirs in the plumbing system of the Popocatepetl Volcanic Complex located from ~30 km b.s.l. (low-Ca clinopyroxene) to ~18 km b.s.l. (T2 clinopyroxene core) and ~10-0 km b.s.l. (T1 clinopyroxene core and rim, T2 clinopyroxene rim) within the crustal structure of the Morelos platform. This provides further support to the complex plumbing system of the Popocatepetl Volcanic Complex consisting of polybaric storage layers of variable interconnected and interacting transient magma reservoirs.

The crystal structure of the mineral montbrayite, a rare gold telluride, was solved using intensity data collected from a crystal of the co-type material from the Robb-Montbray mine, Montbray, Abitibi County, Québec, Canada. The study revealed that the structure is triclinic, space group P1, with cell parameters: a 10.8045(6), b 12.1470(6), c 13.4480(7) Å, ? 108.091(5), ? 104.362(5), ? 97.471(5)8, and V 1583.65(15) Å3. The refinement of an anisotropic model led to an R index of 6.36% for 5145 independent reflections with Fo < 4?(Fo), GooF 1.023. There are 19 Te sites and 12 Au sites in the crystal structure of montbrayite. The Au sites host variable amounts of Sb, whereas all the Te sites but one host only Te. All the Au atoms can be considered to be in octahedral coordination forming more regular AuTe6 or more distorted AuTe4+2 polyhedra, with the exception of one Au position which links five Te atoms and closes its coordination sphere with a contact with itself. Crystalchemical features of the Au and Te atoms are discussed in relation to other gold and silver tellurides. Electron microprobe analyses of the crystal used for the structural study led to the formula Au21.60Sb0.68Te37.41Bi1.31, calculated on the basis of 61 atoms per formula unit. On the basis of information gained from the structural and chemical characterization, the crystalchemical formula was revised, yielding (Au,Ag,Sb,Bi,Pb)23(Te,Sb,Bi,Pb)38 (Z = 1) instead of (Au,Sb)2Te3 (Z = 12) as previously reported. The redefinition of montbrayite has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (voting proposal 17-F). © 2018 Mineralogical Association of Canada. All rights reserved.

Jaszczakite, [(Bi,Pb) 3 S 3 ][AuS 2 ], represents another new species from the abandoned Nagyborzsony Au deposit at Also-Rozsa, northern Hungary. The mineral is closely associated with arsenopyrite, pyrite, marcasite, sphalerite, chalcopyrite, gold, bismuth, bismuthinite, ikunolite, jonassonite, and cosalite, in a matrix of quartz. It is opaque, tin-white, has a metallic lustre and a black streak. Jaszczakite is brittle, and the fracture is irregular. Neither cleavage nor parting are observable in the section. The VHN 5 value ranges between 115 and 132 (mean 122) kg mm −2 which corresponds to a Mohs hardness of 2½–3. The calculated density is 7.327 g cm −3 . In plane-polarized light jaszczakite is weakly bireflectant and pleochroic from light grey to a slightly bluish grey. Internal reflections are absent. The mineral is distinctly anisotropic with rotation tints very similar to those of jonassonite (shades of green and purple). The reflectance COM values (air, R 1 and R 2 in %) are: 49.4, 50.9 (471.1 nm); 47.1, 48.8 (548.3 nm); 46.5, 47.7 (586.6 nm); 48.8, 49.5 (652.3 nm). The average result of five electron-microprobe analyses is Au 19.90(34), Ag 0.22(3), Bi 46.29(54), Pb 16.50(26), Cd 0.11(3), Sb 0.12(4), As 0.08(3), S 16.17(30), Se 0.16(6), total 99.55 wt%. The empirical formula (based on 9 atoms pfu ) is Au 1.00 Ag 0.02 Bi 2.18 Pb 0.78 Cd 0.01 Sb 0.01 As 0.01 S 4.97 Se 0.02 or, according to the refinement results, [(Bi 2.18 Pb 0.78 Ag 0.02 Cd 0.01 Sb 0.01 As 0.01 ) Σ=3.01 S 3 ][AuS 2 ]. The simplified formula is AuBi 3 S 5 , which requires Au 20.01, Bi 63.70, S 16.29, total 100.00 wt%. Jaszczakite is orthorhombic, space group Pmmn , with unit-cell parameters: a = 3.858(1), b = 12.552(3), c = 9.289(2) A, V = 449.83(8) A 3 , a : b : c = 0.3074:1:0.7400, Z = 2. The five strongest observed X-ray powder-diffraction lines [ d in A ( I / I 0 ) ( hkl )] are: 7.466 (100) (011), 6.2760 (90) (020), 5.2003 (24) (021), 3.0963 (68) (003), 2.7768 (26) (023). The crystal structure showed that jaszczakite is the S–Bi-analogue of buckhornite [Pb 2 BiS 3 ][AuTe 2 ]. Jaszczakite is named in honour of John A. Jaszczak, Professor of Physics at the Michigan Technological University, for his significant contributions on the complexities of natural graphite.

Phase relations in the MgSiO3-MgTiO3 and Mg3Al2Si3O12-MgTiO3 systems were studied at 10-24 GPa and 1600 °C using a high-pressure Kawai-type multianvil apparatus. We investigated the full range of starting compositions for the enstatite-geikielite system to derive a P-X phase diagram and synthesize titanium-bearing phases, such as olivine/wadsleyite, rutile, pyroxene, MgTiSi2O7 weberite, bridgmanite and MST-bridgmanite in a wide pressure range. Olivine and pyroxene in run products are characterized by a low titanium content (b0.6 and b0.3 wt% TiO2, respectively) whereas the content of TiO2 in wadsleyite reaches 2 wt% at 12 GPa. The concentration of Ti in MgTiSi2O7 weberite decreases with pressure from 52 wt% TiO2 at 14 GPa to 43 wt% TiO2 at 18 GPa. Two perovskite-type structures (MgSiO3 bridgmanite and Mg(Si,Ti)O3 bridgmanite) were detected in the studied system. MgSiO3 bridgmanite (Brd) is formed at a pressure of N20 GPa and characterized by significant titanium solubility (up to 13 wt% TiO2 at 24 GPa). Mg(Si,Ti)O3 perovskite is formed at a pressure of N17 GPa. The concentration of TiO2 in this phase varies from 29 wt% to 49 wt%. It was found that addition of Ti to the system moves the boundaries of Ol/Wad phase transformations to lower pressures. Addition of Al to the starting material allows us to simulate the composition of natural Ti-rich garnets and bridgmanites. It is important to note that garnet in the Prp-Gkl system is stable throughout a wide pressure range (10-24 GPa). Al incorporation does not affect the distribution of titanium between two types of bridgmanite. It is shown that high contents of Ti stabilize bridgmanite-like compounds at considerably lower pressure than that at the lower mantle/transition zone boundary. Our experiments simulate the composition of natural Ti-rich primary garnet found in eclogite from the Sulu ultrahigh-pressure (UHP) terrane.

Na2MgSiO4 is a mineralogical phase known in the literature as N2MS and reported in several high-pressure experiments (13.6 < P < 22 GPa) to coexist with wadsleyite, garnet and pyroxene. Although the crystal structure of room-PTNa2 Mg2SiO4 is known, a structural study of the phase recovered in high-pressure experiments has not been done yet. Here we present a chemical and structural study of a crystal of Na2MgSiO4 synthesised in the model system Na2MgSiO4-Mg2SiO4 at 22 GPa and 1800 degrees C. The compound was found to crystallize with the orthorhombic cristobalite-type structure previously described at room conditions, space group Pna2(1), with lattice parameters a = 10.8193(8), b = 5.2702(3), c = 7.0598(4) angstrom, V = 402.55(4)angstrom(3), and Z = 4. The structure was refined to R-1 = 0.0184 using 644 observed reflections [F-o > 4 sigma(F-o)] and 73 parameters. The unit-cell of the phase studied here is nearly identical to that of Na2MgSiO4 synthesised at room conditions, thus indicating that it likely relaxed during the pressure release occurring during quenching.

The new results of an experimental study of the majorite MgSiO3–magnesiochromite MgCr2O4 model section are discussed, and general topology of the SiO2–MgO–Cr2O3 system is analyzed. Despite the absence of some petrogenic components (CaO, FeO, Al2O3, Na2O, K2O, and others) in this system, our study, performed in wide pressure range (10–24 GPa), allows us to consider all of the most important phase transformations (in this case, magnesium silicates and oxides) in the upper mantle, transition zone, and uppermost lower mantle. Addition of Cr shows the influence of a minor element on the phase transition parameters. New data on the solubility of Cr in deep minerals (garnet, olivine, wadsleyite, ringwoodite, and bridgmanite) were obtained, which allowed us to determine the influence of Cr on the structural patterns of the major mantle phases. It is shown that addition of 1 wt % Cr2O3 shifts the boundaries of phase transformations by 50 km (olivine/wadsleyite) and 10 km (wadsleyite/ringwoodite) to a lower-pressure domain in comparison with Cr-free systems. In a first approximation, the results of experimental study of phase relations in pseudobinary sections of the SiO2–MgO–Cr2O3 system simulate the phase composition of the restitic part of the upper mantle, transition zone, and uppermost lower mantle under partial melting conditions. It is shown that the Cr concentration in mantle phases is significantly controlled by the Cr/Al ratio in the protolith.

The unique CV-type meteorite Khatyrka is the only natural sample in which "quasicrystals" and associated crystalline Cu,Al-alloys, including khatyrkite and cupalite, have been found. They are suspected to have formed in the early Solar System. To better understand the origin of these exotic phases, and the relationship of Khatyrka to other CV chondrites, we have measured He and Ne in six individual, ~40-��m-sized olivine grains from Khatyrka. We find a cosmic-ray exposure age of about 2-4 Ma (if the meteoroid was, Submitted to Earth and Planetary Science Letters

Shenzhuangite (IMA 2017-018), NiFeS 2 , is a new mineral species found in the shocked Suizhou L6 chondrite. The mineral is closely associated with forsteritic olivine, pyroxene, plagioclase glass (maskelynite), taenite and troilite. It is opaque, and in reflected light shenzhuangite is weakly anisotropic with light brown to greenish rotation tints. Internal reflections are absent. The reflectance COM values (air, R 1 and R 2 in %) are: 24.8, 26.0 (471.1 nm); 34.9, 36.2 (548.3 nm); 37.7, 39.1 (586.6 nm); 40.4, 41.1 (652.3 nm). The average result of 4 electron-microprobe analyses is Ni 22.37(55), Fe 30.87(67), Cu 10.88(22), Co 0.07(4), S 35.42(58), total 99.61 wt%. The empirical formula (based on 4 atoms pfu and assuming the crystal-chemical exchange: Cu + + Fe 3 + ↔ Ni 2 + + Fe 2 + ) is ( Ni 0 . 69 2 + Cu 0 . 31 + ) ( Fe 0 . 69 2 + Fe 0 . 31 3 + ) S 2 . 00 . The simplified formula is NiFeS 2 , which requires Ni 32.85, Fe 31.26, S 35.89, total 100.00 wt%. Shenzhuangite is tetragonal, space group I 4 ¯ 2 d , with unit-cell parameters: a = 5.3121(4), c = 10.4772(7) A, V = 295.65(4) A 3 , c : a = 1.9723, Z = 4. The five strongest observed X-ray powder-diffraction lines [ d in A ( I / I 0 ) ( h k l )] are: 3.05 (100) (1 1 2), 1.875 (20) (2 2 0), 1.591 (25) (3 1 2, 1 1 6), 1.215 (10) (3 3 2, 3 1 6), 1.080 (10) (4 2 4). The crystal structure showed that shenzhuangite is the Ni-analogue of chalcopyrite. It likely formed as alteration of pre-existing taenite when p S 2 / p O 2 ratios allowed sulfurization of the FeNi metal. Shenzhuangite is named in honour of Prof. Shangyue Shen and Prof. Xiaoli Zhuang who first discovered the Ni-rich variety of chalcopyrite in the Suizhou meteorite.

An examination of a specimen of aguilarite from the type locality provides new data on the chemistry and structure of this mineral. The chemical formula of the crystal used for the structural study is (Ag3.98 Cu0.02)(Se0.98 S0.84 Te0.18), on the basis of 6 atoms. The mineral was found to be monoclinic, crystallizing in space group P 21/n, with a = 4.2478(2), b = 6.9432(3), c = 8.0042(5) Å , β = 100.103(2)º, V = 232.41(2) Å3 and Z = 4. The crystal structure [refined to R1 = 0.0139 for 958 reflections with I > 2 σ (I)] is topologically identical to that of acanthite, Ag2S. It can be described as a body-centred array of tetrahedrally coordinated X atoms (X = S, Se and Te) with Ag2X3 triangles in planes nearly parallel to (010); the sheets are linked by the Ag1 silver site, which has twofold coordination.Aguilarite is definitively proved to be isostructural with acanthite; it does not have a naumannite-like structure, as previously supposed. Our data support the hypothesis that there are two solid solution series in the system: a monoclinic 'acanthite-like' series (from Ag2S - Ag2S0.4Se0.6), and an orthorhombic 'naumannite-like' series (from Ag2S0.3Se0.7-Ag2Se). This is supported by data gathered on synthetic counterparts. Aguilarite remains as a valid as a mineral species, but it should be described as the Se-analogue of acanthite.In this study we also (1) review the history of the aguilarite; (2) compare properties of synthetic and natural aguilarite; and (3) demonstrate how earlier researchers erred in describing aguilarite as orthorhombic.The Te-bearing composition of the studied aguilarite crystal suggests the possibility of a solid solution with cervelleite (Ag4TeS).

Phase relations in the system Mg4Si4O12-Mg3Cr2Si3O12 were studied at 10-24 GPa and 1,600 degrees C using a high-pressure Kawai-type multi-anvil apparatus. We investigated the full range of starting compositions for the knorringite-majorite system to derive a P-X phase diagram and synthesize garnets of a wide compositional range. Samples synthesized in the pressure range 10-14 GPa contain knorringite-majorite garnet and Cr-bearing pyroxene. With increasing Cr content in the starting materials, an association of knorringite-majorite garnet and eskolaite is formed. Garnets contain a significant portion of majorite (>10 mol%) even for a pure Mg3Cr2Si3O12 starting composition. Knorringite-majorite garnets were obtained in the pressure range from 10 to 20 GPa. With increasing pressure, the phase assemblages include Cr-bearing MgSiO3 akimotoite and MgSiO3 bridgmanite, as well as MgCr2O4 with calcium titanate structure, and stishovite. Single-crystal X-ray diffraction shows that the incorporation of Cr into the structure of garnet, as well as MgSiO3 akimotoite, and bridgmanite results in an increase in their unit cell parameters. Results of the experimental high-pressure investigation of the pseudo-binary system Mg4Si4O12-Mg3Cr2Si3O12 SiO2-MgO-Cr2O3) may be applied to the origin of high chromium phases mostly garnet) found as inclusions in peridotitic diamonds and formed in bulk rock compositions with high Cr/Al ratios in relation to the primitive mantle.

Mapiquiroite, (Sr,Pb)(U,Y)Fe 2 (Ti,Fe 3+ ) 18 O 38 , was identified as a new mineral from the baryte + pyrite + iron oxide ores of Buca della Vena (BdV) and Monte Arsiccio (MA) mines, Apuan Alps, Tuscany, Italy. At BdV, mapiquiroite occurs as complex rhombohedral or tabular pseudo-hexagonal crystals, up to 1 mm in size, black in colour, with a sub-metallic lustre, in quartz + ‘adularia’ + baryte veins embedded in schists, in association with allanite-(Ce), anatase, destinezite, gypsum, monazite-(Ce), pyrite, rutile, and ‘tourmaline’; at MA, mapiquiroite forms tabular pseudo-hexagonal crystals, up to 5 mm in size, in quartz + carbonate veins embedded in magnetite-rich dolostones, in association with baryte, boulangerite, derbylite, siderite, and sphalerite. Under the microscope, mapiquiroite is blackish in colour, weakly bireflectant, and non-pleochroic. Internal reflections are absent. Anisotropism is distinct, without characteristic rotation tints. Minimum and maximum reflectance data for COM wavelengths [λ (nm), R air (%); data for the specimens from BdV and MA, respectively] are: 471.1, 17.0/17.2 and 17.7/18.0; 548.3, 16.7/17.0 and 17.6/17.8; 586.6, 16.4/16.7 and 17.3/17.7; 652.3, 16.1/16.3 and 17.0/17.3. Vickers hardness is 750 kg · mm −2 and 782 kg·mm −2 for specimens from BdV and MA, respectively, corresponding to a Mohs’ hardness ~ 6. The chemical data point to the formulae (Sr 0.533 La 0.176 Pb 0.105 Na 0.030 Ca 0.026 ) ∑=0.870 (U 0.407 Ce 0.101 Y 0.205 Mn 0.072 ) ∑=0.785 Fe 3+ 2.000 (Ti 12.423 Fe 3+ 3.345 Cr 1.624 V 5+ 0.129 Al 0.032 Sn 0.013 Nb 0.007 ) ∑=17.573 O 38 (BdV) and (Sr 0.312 Pb 0.248 Na 0.019 Ca 0.014 La 0.009 ) ∑=0.602 (U 0.858 Y 0.070 Ce 0.021 Mn 0.005 ) ∑=0.954 (Fe 3+ 1.695 Zn 0.305 ) ∑=2.000 (Ti 12.070 Fe 3+ 4.987 V 5+ 0.372 Al 0.030 Nb 0.030 Cr 0.015 Sn 0.001 ) ∑=17.505 O 38 (MA). Mapiquiroite is trigonal, R 3, with a 10.3719(7), c 20.875(1) A, V 1944.8(2) A 3 and a 10.3854(3), c 20.8942(6) A, V 1951.7(1) A 3 for the specimens from BdV and MA, respectively. The crystal structures of mapiquiroite from these two occurrences were refined to R 1 = 0.028 and 0.032, respectively. Mapiquiroite is isostructural with other members of the crichtonite group. The name mapiquiroite honours four Italian mineral collectors, Riccardo Mazzanti, Luigi Pierotti, Ugo Quilici, and Moreno Romani for their contribution to the study of the mineralogy of baryte + pyrite + iron oxide deposits from the Apuan Alps.

Na-majorite Na 2 MgSi 5 O 12 (NaMaj), an end-member of sodium-rich majoritic garnet, was synthesized and garnet/pyroxene transition was studied in Kawai-type multi-anvil experiments at 11–20 GPa and 1500–2300 °C. Na-majorite was obtained at 16 GPa and 1500 °C; its stability spreads to the high-temperature region with pressure (1900 °C at 17 GPa and 2100 °C at 19.5 GPa). Experimental study of the Mg 3 Al 2 Si 3 O 12 (Prp)–Na 2 MgSi 5 O 12 join at 11–20 GPa demonstrated the following phase assemblages changing with pressure: garnet + two pyroxenes (enstatite-rich and Na-pyroxene); garnet + pyroxene (enstatite–jadeite–Na-pyroxene) + stishovite; garnet + stishovite; garnet + NaAlSiO 4 with calcium ferrite-type structure + stishovite. For a given bulk composition (Prp 50 NaMaj 50 ), a regular increase of both Na and Si concentrations and consequently, NaMaj mole portion in garnets was observed. From 8.5 to 20 GPa, the concentration of Na 2 O increased from 1.52 to 5.71 wt.%, which correspond to a NaMaj content of ∼40 mol.%. The highest Na 2 O content (12.24 wt.%) was registered at 18.5 GPa and 1600 °C for the Prp 20 NaMaj 80 starting composition. Experiments at 18.5 GPa and 1600 °C on the pyrope–Na-majorite join allowed us to study mixing peculiarities for sodium-rich majoritic garnet. The transition from cubic ( Ia 3 d ) to tetragonal ( I 4 1 / acd ) symmetry was observed for the composition with ∼80 mol.% Na 2 MgSi 5 O 12 , which is consistent with the similar change of the structure in the pyrope–majorite (Mg 4 Si 4 O 12 ) system. Significant Na-majorite solubility in pyrope, as well as findings of natural garnets with high Na concentrations (>1 wt.% Na 2 O) allow us to consider Na-bearing majoritic garnet as an important concentrator of sodium in the deep upper mantle and transition zone.

Tondiite, with the simplified formula Cu3Mg(OH)6Cl2, occurs as a rare supergene mineral in a phonolitic tephrite from the type locality, Vesuvius volcano, Italy, as well as associated with haydeeite in the Santo Domingo Mine, Arica Province, Chile. It is emerald green to bright green in colour and occurs in irregularly shaped crystals, often with stepped faces. Its calculated density is 3.503 g cm−3. Tondiite crystallizes with the herbertsmithite structure type, space group Rm. Lattice parameters are a = 6.8377(7) Å and c = 14.088(2)Å for the holotype material. The c parameter may vary with Mg/Cu ratio and the presence of impurity atoms. The five strongest lines in the calculated powder diffraction pattern are [d in Å(I)(hkil)]: 5.459(88)(101), 3.419(22)(110), 2.764(100)(112 3), 2.266(54)(024), 1.706(26)(220). Several tondiite crystals have been examined by single-crystal X-ray diffraction and by electron microprobe analysis. The observed Mg content ranges between 0.6 and 0.7 atoms per formula unit. The structural role of Mg is discussed.

This paper reports on the occurrence and the crystal structure of kircherite, a new member of the cancrinite-sodalite group of minerals from Valle Biachella, Sacrofano community (Rome, Latium, Italy). The mineral occurs in association with sodalite, biotite, iron oxides, titanite, fluorite, and a pyrochlore-group mineral. The groundmass of the ejectum consists essentially of K-feldspar with subordinate plagioclase. Kircherite (3 mm as largest size) is observed within miarolitic cavities of the rock and typically occurs as parallel associations of hexagonal, thin, tabular colorless to light-gray transparent crystals; it is non-pleochroic and uniaxial negative, with ω = 1.510(2) and ɛ = 1.502(2). D calc is 2.457 g/cm 3 . Kircherite is trigonal with a = 12.8770(7), c = 95.244(6) A, V = 13677(1) A 3 , Z = 1. The structure has been refined in the trigonal space group R 32, obtaining a R -value of 8.5% on 8131 reflections with I /σ I > 2. The strongest seven reflections in the X-ray powder pattern are [ d in A (I %) ( hkl )]: 3.717 (100) (3 0 0), 2.648 (100) (2 1 28; 0 0 36), 3.232 (65) (2 1 19), 3.584 (60) (1 2 14), 3.604 (53) (1 0 25), 3.799 (52) (1 2 11), 3.220 (38) (2 2 0). The single-crystal FTIR spectrum rules out OH groups and shows the presence of H 2 O and CO 2 molecules in the structural cages of the mineral. Chemical analysis gives (in wt%): SiO 2 32.05, Al 2 O 3 27.13, FeO 0.07, K 2 O 4.38, CaO 8.75, Na 2 O 13.62, MgO 0.01, MnO 0.02, TiO 2 0.01, SO 3 12.87, Cl 0.35, F 0.05, total 99.82. The empirical formula calculated on the basis of ∑(Si+Al) = 216 apfu is (Na 89.09 Ca 31.63 K 18.85 Fe 0.20 Mn 0.06 Mg 0.05 Ti 0.03 ) ∑=139.91 [(Si 108.13 Al 107.87 ) ∑=216.00 O 430.00 ](SO 4 ) 32.58 Cl 2.00 F 0.53 ·6.86H 2 O, which corresponds to the ideal formula [Na 90 Ca 36 K 18 ] ∑=144 (Si 108 Al 108 O 432 )(SO 4 ) 36 ·6H 2 O. The structure can be described as a stacking sequence of 36 layers of six-membered rings of tetrahedra along the c axis. The stacking sequence is ACABCABCABCACBCABCABCABCBABCABCABCAB…, where A, B, and C represent the positions of the rings within the layers. This sequence gives rise to cancrinite, sodalite, and losod cages, alternating along c . Sulfate groups occur within the sodalite and losod cages associated by Na, K, and Ca. H 2 O groups occur within the cancrinite cages, bonded to Ca and Na cations. Anion groups (SO 4 2− ) in sodalite cages show positional disorder, and so do consequently the extraframework cation sites related to them.

Pyrochlore-group minerals that exhibit high Si contents are fairly common in geochemically evolved parageneses. However, the role of Si in the structure of these minerals is unclear. Different explanations have been invoked to clarify the way in which Si is incorporated in natural pyrochlores. These include the presence of impurities, the presence of Si in an amorphous or dispersed state, and its presence as an essential part of the structure. This paper reports an integrated XREF, SEM, EMPA, and TEM study on pyrochlore samples with high SiO 2 content (up to 11.51 wt%) from Narssârssuk, Julianehaab district (Greenland). TEM observations reveal that Si-poor areas have strong and sharp diffraction peaks, whereas the Si-rich areas showed weaker spots with the diffuse diffraction halo typical of a metamict material. No evidence of crystalline phases other than pyrochlore was observed. Two single crystals having the unit-cell parameter a = 10.4200(7) and 10.3738(7) A, respectively, were analyzed by X-ray diffraction and the structure was refined to R obs = 2.62 and 4.35%. On the basis of both the refined site scattering and the octahedral bond distance and the results of the TEM investigation, only a fraction (~30–50%) of the Si detected by EMPA is incorporated in the structure. A comparison with structural data of Si-free pyrochlores reported in the literature supports this assumption and allows a linear multiple regression to model the effect of the substitution of (Nb,Ta) by Ti and Si. The remaining 50–70% of the total silicon detected is incorporated in the radiation-damaged portions of pyrochlore.