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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 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.

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.

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).

An in situ high-pressure X-ray diffraction study has been carried out at room temperature up to 9.26 GPa on synthetic Mn2Sb2O7 having a weberite-3T structure. A 2nd-order Birch–Murnaghan Equation of State (EoS) was used to refine the pressure–volume data. The refinement of the unit-cell volume and of the isothermal bulk modulus at room pressure leads to: V0 = 782.7(2), KT0 = 150(1) GPa. Unit-cell parameters decrease gradually as a function of pressure with a bulk modulus anisotropy scheme, with a being the softest direction. The overall mean polyhedral distances are quite constant, indicating a scarce compressibility of both the A and B polyhedra in the pressure range investigated. The compressional behaviour of Mn2Sb2O7 is compared with that shown by ingersonite, Ca3MnSb4O14, and synthetic orthorhombic Ca2Sb2O7.

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.