Crystal Structure of Natural Non-metamict Ti- and Fe~(2+)-rich Chevkinite-(Ce)

1 Introduction Chevkinite groups can be assigned to the chevkinite-(Ce) subgroup and perrierite-(Ce) subgroup in accord with the angle β : β ≈ 100o for the chevkinite subgroup and β ≈ 113o for the perrierite subgroup. Chevkinite-(Ce), polykovite-(Ce) and Maoniupingite (new mineral No. 017 of 2003) belong to the former subgroup, while renjeite and matsubaraite belong to the latter group. As strontio-chevkinite is a Sr-analogue of perrierite, usually the natural chevkinite-(Ce) group min…     

The Crystal Structure of Lisiguangite

The crystal structure of lisiguangite,CuPtBiS3,from Yanshan mountains,Chengde Prefecture,Hebei Province,China has been determined by single crystal X-ray diffraction.It belongs to orthorhombic space group P2_12_12_1 with a = 7.7372(15) A,b = 12.844(3) A,c = 4.9062(10) A,V =487.57(17) A~3,Z = 4.The final full-matric least-square refinement on F2 converged with Rl = 0.0495 and wR2 = 0.0992 for 704 observed reflections[I≥2σ(I)].Lisiguangite is the isomorph of known CuNiSbS_3 and CuNiBiS_3· Pt~(2+) and Bi~(3+) have the distorted octahedral coordination enviroments composed of two metal and four S and Cu~(+2) has a distorted tetrahedral coordination environment with four S atoms.Each S atom is surrounded by four metals to give a tetrahedral environment.The crystal structure is a complex 3 dimensional network.     

Crystal Structure and Chemical Composition of Ludwigite from Vranovac Ore Deposit (Boranja Mountain,Serbia)

The crystal structure of ludwigite from Vranovac ore deposit (Boranja Mt., Serbia) was refined using the X-ray powder diffraction (XRPD) Rietveld method in the space group Pbam to a final RB=7.45% and RF=5.26%. It has the unit cell dimensions of: a=9.2515(2) ?; b=12.3109(2) ?; c=3.03712(7) ?; and V=345.91(1) ?3. The calculated distances and angles are mostly in good agreement with the Mg2+-Fe2+ substitutions across the M(1) and M(3) sites, as well as with the Fe3+-Al3+ replacement in the M(4) site. However, the mean observed M(2)-O distance is considerably shorter than prescribed, due to a slight increase of the Fe3+ content in the M(2) site. Such replacement was compensated by slight increase of the Fe2+ content in the M(4) site, resulting in the (Mg1.48Fe2+0.46Fe3+0.05Mn0.02)2.01(Fe3+0.94Fe2+0.04Al0.02)1.00B1.00O5 composition. The formation temperature was estimated to be about 500–600°C. The influences of the various chemical compositions to the crystallographic parameters, M-O distances, M(3) and M(4) sites shift, distortion parameters and estimated valences, were also studied and compared with other reference samples.     

Lisiguangite,CuPtBiS3, a New Platinum‐Group Mineral from the Yanshan Mountains,Hebei, China

Lisiguangite, CuPtBiS3, is a new mineral species discovered in a PEG-bearing, Co-Cu sulfide vein in garnet pyroxenite of the Yanshan Mountains, Chengde Prefecture, Hebei Province, China. It is associated with chalcopyrite and bornite, galena, minor pyrite, carrolite, molybdenite and the platinum-group minerals daomanite (CuPtAsS2), Co-bearing malanite (Cu(Pt, Co)2S4) sperrylite, moncheite, cooperite and malyshevite (CuPdBiS3), rare damiaoite (Pt2In3) and yixunite (Pt3In). Lisiguangite occurs as idiomorphic crystals, tabular or lamellae (010) and elongated [100] or as aggregates, up to 2 mm long and 0.5 mm wide. The mineral is opaque, has lead-gray color, black streak and metallic luster. The mineral is non-fluorescent. The observed morphology displays the following forms: pinacoids {100}, {010}, {001}, and prism {110}. No twining is observed. The a:b:c ratio, calculated from unit-cell parameters, is 0.6010:1:0.3836. Cleavage: {010} perfect, {001} distinct, {100} may be visible. H Mohs: 21/2; VHN25=46.7-49.8 (mean 48.3) kg/mm2. Tenacity: brittle. Lisiguangite is bright white with a yellowish tint. In reflected light it shows neither internal reflections nor bireflectance or pleochroism. It has weak to moderate anisotropy (blue-greenish to brownish) and parallel-axial extinction. The reflectance values in air (and in oil) for R3, R4 and (imR3, imR4), at the standard Commission on Ore Mineralogy wavelengths are: 37.5, 35.7 (23.4, 22.3) at 470 nm; 38.6, 36.5 (23.6, 22.6) at 546 nm; 39.4, 37.5 (23.6, 22.7) at 589 nm and 40.3, 38.2 (23.7, 22.9) at 650 nm. The average of eight electron-microprobe analyses: Cu 12.98, Pt 30.04, Pd 2.69, Bi 37.65 and S 17.55, totaling 100.91%, corresponding to Cu1.10(Pt 0.83, Pd0.14)Σ0.97Bi0.97S2.96 based on six atoms apfu. The ideal formula is CuPtBiS3. The mineral is orthorhombic. Space group: P212121, a=7.7152(15)?,b=12.838(3)?, c=4.9248(10)?, V=487.80(17)?3, Z=4. The six strongest lines in the X-ray powder-diffraction pattern [d in ? (I) (h k l) are 6.40(30)(020), 3.24(80)(031), 3.03(100)(201), 2.27(40)(051), 2.14(50)(250), 1.865(60)(232).     

Synthesis,crystallographic data,solubility and electrokinetic properties of meta-zeunerite,meta-kirchheimerite and nickel-uranylarsenate

{M[UO₂¦AsO₄]₂ · nH₂O} with M=Cu²⁺, Co²⁺, Ni²⁺ has been synthesized from reagent grade chemicals and by ion exchange of trögerite {HUO₂AsO₄ · 4 H₂O}. Synthetic meta-zeunerite (M=Cu²⁺), meta-kirchheimerite (M=Co²⁺) and nickel-uranylarsenate are all tetragonal. The cell parameters determined from Guinier-Hägg diffraction data for {Cu[UO₂¦AsO₄]₂ · 8 H₂O} are a=b=7.10 Å and c=17.42 Å, with Z=2 and the measured density 3.70 g cm^?3. The cell parameters for {Co[UO₂¦AsO₄]₂ · 7 H₂O} and {Ni[UO²¦AsO⁴]² · 7 H₂O} are a=b=20.25 Å and c=17.20 Å, with Z=16 and the measured density 3.82 and 3.74 g cm^?3, respectively. The solubility products for synthetic Cu-, Co- and Ni-uranylarsenate at 25° C are 10^?49.20, 10^?45.34 and 10^?45.10, respectively. The zeta-potential remains negative between pH=2 and pH=9 and is strongly affected by the presence of different cations.     

A New Three–Dimensional Superstructure in Bafertisite

A new superstructure was found in bafertisite [(Ba0.98Na0.02)1.00(Fe1.71Mn0.26Mg0.01)1.98 TiO[(Si1.82Ti0.04Al0.03Cr0.01)1.90O7](OH1.40F0.53Cl0.03)1.96] from Donghai County, Jiangsu Province, China. The occurrence of the superstructure reflections were observed by single crystal diffraction using a SMAR APEX CCD. The a*, b*and c* axis directions revealed extra weak reflection spots of the superstructure. The apparent 2a, 2b and 2c superstructure is monoclinic with unit cell a＝10.6502(15)?, b＝13.7233(19)?, c＝21.6897(3)?, α＝90o, β＝94.698(3)o, γ＝90o,space group Cm,Z=16. If c* extra weak reflections are ignored, the secondary supercell gave a cell a＝10.6548(15)?, b＝13.7284(19)?, c＝11.6900(17)?, α＝90o, β＝112.322(28)o, γ＝90o,space group Cm,Z=8. The basic subcell was obtained by ignoring all extra weak reflection spots and gave: a＝5.3249(17)?, b＝6.8669(22)?, c＝10.8709(36)?, α＝90o, β＝94.740(62)o, γ＝90o,space P21/m,Z=2. The superstructure has been refined to R = 0.063 for 7805 [R(int) = 0.0266] unique reflections I>2δ(I). The structure consists of an octahedra (O) sheet sandwiched between two heteropolyhedral (H) sheets. These sheets consist of Ti–octahedra and twin tetrahedral disilicate groups [Si2O7]. The O sheet comprises (Fe,Mg)O4 octahedra. The large Ba cation is located in the interlayer area. The refined structure shows Fe, Mg are partly ordered. The shifting of the TiO6 octahedron and SiO4 tetrahedron sites in the sheet may be a consequence of the superstructure.     

Structural investigation of the synthetic CaAn(PO4)2 (An = Th and Np) cheralite-like phosphates

The crystallographic structures of the synthetic cheralite, CaTh(PO₄)₂, and its homolog CaNp(PO₄)₂ have been investigated by X-ray diffraction at room temperature. Rietveld analyses showed that both compounds crystallize in the monoclinic system and are isostructural to monazite LnPO₄ (Ln = La to Gd). The space group is P2₁/n (I.T. = 14) with Z = 2. The refined lattice parameters of CaTh(PO₄)₂ are a = 6.7085(8) Å, b = 6.9160(6) Å, c = 6.4152(6) Å, and β = 103.71(1)° with best fit parameters R _wp = 4.87%, R _p = 3.69% and R _B = 3.99%. For CaNp(PO₄)₂, we obtained a = 6.6509(5) Å, b = 6.8390(3) Å, c = 6.3537(8) Å, and β = 104.12(6)° and R _wp = 6.74%, R _p = 5.23%, and R _B = 6.05%. The results indicate significant distortions of bond length and angles of the PO₄ tetrahedra in CaTh(PO₄)₂ and to a lesser extent in CaNp(PO₄)₂. The structural distortions were confirmed by Raman spectroscopy of CaTh(PO₄)₂. A comparison with the isostructural compounds LnPO₄ (Ln = Ce and Sm) confirmed that the substitution of the large rare earth trivalent cations with Ca²⁺ and Th⁴⁺ introduces a distortion of the PO₄ tetrahedra.     

Sodium trisilicate: A new high-pressure silicate structure (Na2Si[Si2O7])

Crystals of sodium trisilicate (Na₂Si₃O₇) have been grown in the presence of melt at 9 GPa, 1200 °C using the MA6/8 superpress at Edmonton, and the X-ray structure determined at room pressure (R=2.0%). Na₂Si₃O₇ is monoclinic with a=8.922(2) Å, b= 4.8490(5) Å, c=11.567(1) Å, β=102.64(1)° (C2/c), D _x = 3.295 g·cm^-3. Silicon occurs in both tetrahedral and octahedral coordination (^[6]Si∶^[4]Si = l∶2). The SiO₄ tetrahedra form a diorthosilicate [Si₂O₇] group and are linked by the isolated SiO₆ octahedra via shared corners into a framework of 6-membered (^[4]Si-^[4]Si-^[6]Si^[4]Si-^[4] Si-^[6]Si) and 4-membered (^[4]Si-^[6]Si-^[4]Sr-^[6]Si) rings: 〈^[6]Si-O〉=1.789 Å, 〈^[4]Si-O〉= 1.625 Å, ^[4]Si-O-^[4]Si=132.9° and the bridging oxygen is overbonded (s = 2.22). Channels parallel to b-axis and [110] accommodate Na in irregular 6-fold coordination: 〈Na-O〉 = 2.511 Å.     

Crystal chemical characteristics of α-CaSi2O5, a new high pressure calcium silicate with five-coordinated silicon synthesized at 1500°C and 10 GPa

The crystal structure of α-CaSi₂O₅ synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P , using single crystal X-ray diffraction data. The composition (Z=4) and unit cell are Ca_1.02Si_1.99O₅ by EPMA analysis and a=7.243(2) Å, b=7.546(4) Å, c=6.501(4) Å, α=81.43(5)°, β=84.82(4)°, γ=69.60(3)°, V=329.5(3) ų, yielding the density value, 3.55 g/cm³. The structure is closely related to that of titanite, CaTiSiO₅ and features the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 Å. The substantial deviation of valence sum for Ca indicates the existence of local strain and the instability of α-CaSi₂O₅ at room pressure.     

High pressure single-crystal synthesis, structure and compressibility of the garnet Mn2+ 3Mn3+ 2[SiO4]3

Single crystals of the garnet Mn²⁺ ₃Mn³⁺ ₂[SiO₄]₃ and coesite were synthesised from MnO₂-SiO₂ oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3¯d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca₃Mn³⁺ ₂[GeO₄]₃ (space group I4₁/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn³⁺. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn³⁺-O=1.995 (4) Å (octahedron), Mn²⁺-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K [a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K ₀=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn²⁺ ₃Mn³⁺ ₂[SiO₄]₃ were assigned based on a calderite Mn²⁺ ₃Fe³⁺ ₂[SiO₄]₃ model extrapolated from andradite and grossular literature data.     

氟碳铈钡矿(Cebaite)的新资料

氟碳铈钡矿产于白云鄂博铁铌稀土矿床中。1965年发现于西矿区。贵阳地球化学所稀有矿物研究组(1972)和彭志忠、沈今川等(1980)及后来李方华等对该矿物进行了矿物学研究。本文报道了采自东矿区几个样品的分析测定结果。 一号氟碳铈钡矿(样品编号:东1606),产于东矿体靠近上盘的钠辉石型矿石中,与钠辉石、萤石、重晶石等矿物共生,矿物呈粒状或板状,大小不一,集合体大者直径可达数毫米。     

Theoretical Prediction of Gibbs Free Energies of Formation for Crystalline α-MOOH and α-M2O3 Based on a Linear Free-Energy Relationship

In the present study, the modified Sverjensky–Molling equation, derived from a linear-free energy relationship, is used to predict the Gibbs free energies of formation of crystalline phases of α-MOOH (with a goethite structure) and α-M2O3 (with a hematite structure) from the known thermodynamic properties of the corresponding aqueous trivalent cations (M3+). The modified equation is expressed as ΔG0f,MVX=aMVXΔG0n,M3++bMVX+βMVXγM3+, where the coefficients aMVX, bMVX, and βMVX characterize a particular structural family of MvX (M is a trivalent cation [M3+] and X represents the remainder of the composition of solid); γ3+ is the ionic radius of trivalent cations (M3+); ΔG0f,MVX is the standard Gibbs free energy of formation of MvX; and ΔG0n,M3+ is the non-solvation energy of trivalent cations (M3+). By fitting the equation to the known experimental thermodynamic data, the coefficients for the goethite family (α-MOOH) are aMVX=0.8838, bMVX=?424.4431 (kcal/mol), and βMVX=115 (kcal/mol.?), while the coefficients for the hematite family (α-M2O3) are aMVX=1.7468, bMVX=?814.9573 (kcal/mol), and βMVX=278 (kcal/mol.?). The constrained relationship can be used to predict the standard Gibbs free energies of formation of crystalline phases and fictive phases (i.e. phases that are thermodynamically unstable and do not occur at standard conditions) within the isostructural families of goethite (α-MOOH) and hematite (α-M2O3) if the standard Gibbs free energies of formation of the trivalent cations are known.     

Cation substitution in synthetic meridianiite (MgSO<Subscript>4</Subscript>·11H<Subscript>2</Subscript>O) I: X-ray powder diffraction analysis of quenched polycrystalline aggregates

Meridianiite, MgSO₄·11H₂O, is the most highly hydrated phase in the binary MgSO₄–H₂O system. Lower hydrates in the MgSO₄–H₂O system have end-member analogues containing alternative divalent metal cations (Ni²⁺, Zn²⁺, Mn²⁺, Cu²⁺, Fe²⁺, and Co²⁺) and exhibit extensive solid solution with MgSO₄ and with one another, but no other undecahydrate is known. We have prepared aqueous MgSO₄ solutions doped with these other cations in proportions up to and including the pure end-members. These liquids have been solidified into fine-grained polycrystalline blocks of metal sulfate hydrate + ice by rapid quenching in liquid nitrogen. The solid products have been characterised by X-ray powder diffraction, and the onset of partial melting has been quantified using a thermal probe. We have established that of the seven end-member metal sulfates studied, only MgSO₄ forms an undecahydrate; ZnSO₄ forms an orthorhombic heptahydrate (synthetic goslarite), MnSO₄, FeSO₄, and CoSO₄ form monoclinic heptahydrates (syn. mallardite, melanterite, bieberite, respectively), and CuSO₄ crystallises as the well-known triclinic pentahydrate (syn. chalcanthite). NiSO₄ forms a new hydrate which has been indexed with a triclinic unit cell of dimensions a = 6.1275(1) Å, b = 6.8628(1) Å, c = 12.6318(2) Å, α = 92.904(2)°, β = 97.678(2)°, and γ = 96.618(2)°. The unit-cell volume of this crystal, V = 521.74(1) ų, is consistent with it being an octahydrate, NiSO₄·8H₂O. Further analysis of doped specimens has shown that synthetic meridianiite is able to accommodate significant quantities of foreign cations in its structure; of the order 50 mol. % Co²⁺ or Mn²⁺, 20–30 mol. % Ni²⁺ or Zn²⁺, but less than 10 mol. % of Cu²⁺ or Fe²⁺. In three of the systems we examined, an ‘intermediate’ phase occurred that differed in hydration state both from the Mg-bearing meridianiite end-member and the pure dopant end-member hydrate. In the case of CuSO₄, we observed a melanterite-structured heptahydrate at Cu/(Cu + Mg) = 0.5, which we identify as synthetic alpersite [(Mg_0.5Cu_0.5)SO₄·7H₂O)]. In the NiSO₄- and ZnSO₄-doped systems we characterised an entirely new hydrate which could also be identified to a lesser degree in the CuSO₄- and the FeSO₄-doped systems. The Ni-doped substance has been indexed with a monoclinic unit-cell of dimensions a = 6.7488(2) Å, b = 11.9613(4) Å, c = 14.6321(5) Å, and β = 95.047(3)°, systematic absences being indicative of space-group P2₁/c with Z = 4. The unit-cell volume, V = 1,176.59(5) ų, is consistent with it being an enneahydrate [i.e. (Mg_0.5Ni_0.5)SO₄·9H₂O)]. Similarly, the new Zn-bearing enneahydrate has refined unit cell dimensions of a = 6.7555(3) Å, b = 11.9834(5) Å, c = 14.6666(8) Å, β = 95.020(4)°, V = 1,182.77(7) ų, and the new Fe-bearing enneahydrate has refined unit cell dimensions of a = 6.7726(3) Å, b = 12.0077(3) Å, c = 14.6920(5) Å, β = 95.037(3)°, and V = 1,190.20(6) ų. The observation that synthetic meridianiite can form in the presence of, and accommodate significant quantities of other ions increases the likelihood that this mineral will occur naturally on Mars—and elsewhere in the outer solar system—in metalliferous brines.     

Titanium,Ti, A New Mineral Species from Luobusha,Tibet, China

We describe the new mineral species titanium,ideally Ti,found in the podiform chromitites of the Luobusha ophiolite in Tibet,People’s Republic of China.The irregular crystals range from 0.1 to 0.6 mm in diameter and form an intergrowth with coesite and kyanite.Titanium is silver grey in colour,the luster is metallic,it is opaque,the streak is grayish black,and it is non-fluorescent.The mineral is malleable,has a rough to hackly fracture and has no apparent cleavage.The estimated Mohs hardness is 4,and the calculated density is 4.503 g/cm3.The composition is Ti 99.23-100.00 wt%.The mineral is hexagonal,space group P63 /mmc.Unit-cell parameters are a 2.950(2),c 4.686(1),V 35.32(5) 3,Z = 2.The five strongest powder diffraction lines [d in(hkl)(I/I0)] are: 2.569(010)(32),2.254(011)(100),1.730(012)(16),1.478(110)(21),and 0.9464(121)(8).The species and name were approved by the CNMNC(IMA 2010–044).     

Effects of temperature on the crystal structure of epidote: a neutron single-crystal diffraction study at 293 and 1,070 K

The effects of temperature on the crystal structure of a natural epidote [Ca_1.925 Fe_0.745Al_2.265Ti_0.004Si_3.037O₁₂(OH), a = 8.890(6), b = 5.630(4), c = 10.150(6) Å and β = 115.36(5)°, Sp. Gr. P2₁ /m] have been investigated by means of neutron single-crystal diffraction at 293 and 1,070 K. At room conditions, the structural refinement confirms the presence of Fe³⁺ at the M₃ site [%Fe(M3) = 73.1(8)%] and all attempts to refine the amount of Fe at the M(1) site were unsuccessful. Only one independent proton site was located. Two possible hydrogen bonds, with O(2) and O(4) as acceptors [i.e. O(10)–H(1)···O(2) and O(10)–H(1)···O(4)], occur. However, the topological configuration of the bonds suggests that the O(10)–H(1)···O(4) is energetically more favourable, as H(1)···O(4) = 1.9731(28) Å, O(10)···O(4) = 2.9318(22) Å and O(10)–H(1)···O4 = 166.7(2)°, whereas H(1)···O(2) = 2.5921(23) Å, O(10)···O(2) = 2.8221(17) Å and O(10)–H(1)···O2 = 93.3(1)°. The O(10)–H(1) bond distance corrected for “riding motion” is 0.9943 Å. The diffraction data at 1,070 K show that epidote is stable within the T-range investigated, and that its crystallinity is maintained. A positive thermal expansion is observed along all the three crystallographic axes. At 1,070 K the structural refinement again shows that Fe³⁺ share the M(3) site along with Al³⁺ [%Fe(M3)_1,070K = 74(2)%]. The refined amount of Fe³⁺ at the M(1) is not significant [%Fe(M1)_1,070K = 1(2)%]. The tetrahedral and octahedral bond distances and angles show a slight distortion of the polyhedra at high-T, but a significant increase of the bond distances compared to those at room temperature is observed, especially for bond distances corrected for “rigid body motions”. The high-T conditions also affect the inter-polyhedral configurations: the bridging angle Si(2)–O(9)–Si(1) of the Si₂O₇ group increases significantly with T. The high-T structure refinement shows that no dehydration effect occurs at least within the T-range investigated. The configuration of the H-bonding is basically maintained with temperature. However, the hydrogen bond strength changes at 1,070 K, as the O(10)···O(4) and H(1)···O(4) distances are slightly longer than those at 293 K. The anisotropic displacement parameters of the proton site are significantly larger than those at room condition. Reasons for the thermal stability of epidote up to 1,070 K observed in this study, the absence of dehydration and/or non-convergent ordering of Al and Fe³⁺ between different octahedral sites and/or convergent ordering on M(3) are discussed.     

Crystal structure of xinganite

Xinganite is a new REE-Be-rich silicate discovered in China. Its ideal formula is: (Y, Ce)Be SiO₄ OH. The mineral is of monoclinic system. The intensity data were collected with a single-crystal four-circle diffractometer. The lattice parameters are: a=4.7681 (± 0.00263) Å,b=7.7657 (± 0.00686) Å, c=9.9301 (± 0.00639) Å; α =90°, β=90.171° (±0.0053°), γ=90° space group p2¹/c;,Z=4. The crystal structure has been determined by direct methods and electron density synthesis methods. The least squares refinement gave a final discrepancy indexR=0.086. The crystal structural analysis shows that xinganite is of datolite-type structure.     

Refinement of the crystal structure of fornacite using the rietveld method

The crystal structure of fornacite Pb₂(Cu,Fe)[CrO₄(As,P)O₄OH] from the Berezovskii deposit (Central Urals, Russia) was refined by X-ray powder diffraction data using the Rietveld method. Fornacite is monoclinic, space group P2₁/c, the unit cell dimensions are a = 8.09015(12), b = 5.90913(9), c = 17.4839(2) Å, β = 109.99(2), V = 785.5(3) ų, and Z = 4. The structure was refined in the isotropic approximation of the atomic displacement parameters up to R _p = 0.0516, R _wp = 0.0692, R _B = 0.0229, and R _F = 0.0200. The fornacite structure is similar to that of minerals of the brackebuschite-group and consists of heteropolyhedral chains, built by the columns of edge-sharing Cu²⁺O₆ octahedra connected with isolated Cr⁶⁺O₄ and As⁵⁺O₄ tetrahedra. The chains are linked by ninefold Pb²⁺ polyhedra.     

Arcanite from Fumarole Exhalations of the Tolbachik Volcano (Kamchatka,Russia) and Its Crystal Structure

The crystal structure (R = 0.0194) of arcanite β-K₂SO₄ was studied on a single crystal from exhalations of the Arsenatnaya fumarole, Tolbachik Volcano (Kamchatka, Russia). The mineral crystallizes at a temperature of ≥350–430°C and associates with langbeinite, aphthitalite, hematite, tenorite, johillerite, and others. Arcanite is orthorhombic, Pnma, a = 7.4763(2) Å, b = 5.77262(16) Å, c = 10.0630(3) Å, V = 434.30(2) ų, Z = 4. Its structure contains isolated SO₄ tetrahedra, whereas K cations center ten- and nine-fold polyhedra.     

Crystal chemistry of natural and synthetic lead oxohalides: II. Crystal structure of Pb7O4(OH)4Br2

Crystals of lead oxobromide Pb₇O₄(OH)₄Br₂ have been synthesized by hydrothermal method. The structure of the new compound has been studied with X-ray single-crystal diffraction analysis. The compound is monoclinic, space group C112₁; unit-cell dimensions are a = 5.852(4), b = 13.452(7), c = 19.673(9) Å, γ = 90.04°, V = 1548.7(15) ų. The structure has been solved by direct methods and refined to R ₁ = 0.1138 for 1847 observed Pb₇O₄(OH)₄Br₂ unique reflections. The structure contains seven symmetrically independent bivalent Pb atoms. The coordination polyhedrons of Pb are strongly distorted due to stereochemical activity of unshared electron pair 6s ². Oxygen atoms are tetrahedrally coordinated by four Pb²⁺ cations with the formation of oxocentered tetrahedrons OPb₄. The compound is based on [O₂Pb₃]²⁺ double chains formed by OPb₄ tetrahedrons. (OH)Pb₂ dimers combine the [O₂Pb₃]²⁺ chains into 3D framework. Channels in the framework are parallel to [100] and are occupied by Br anions.     

Yarlongite: A New Metallic Carbide Mineral

Yarlongite occurs in ophiolitic chromitite at the Luobusha mine (29°5′N 92°5′E, about 200 km ESE of Lhasa), Qusum County, Shannan Prefecture, Tibet Autonomous Region, People’s Republic of China. Associated minerals are: diamond, moissanite, wüstite, iridium (“osmiridium”), osmium (“iridosmine”), periclase, chromite, native iron, native nickel, native chromium, forsterite, Cr-rich diopside, intermetallic compounds Ni-Fe-Cr, Ni-Cr, Cr-C, etc. Yarlongite and its associated minerals were handpicked from a large heavy mineral sample of chromitite. The metallic carbides associated with yarlongite are cohenite, tongbaite, khamrabaevite and qusongite (IMA2007-034). Yarlongite occurs as irregular grains, with a size between 0.02 and 0.06 mm, steel-grey colour, H Mohs: 5?-6. Tenacity: brittle. Cleavage: {0 0 1} perfect. Fracture: conchoidal. Chemical formula: (Cr4Fe4Ni)Σ9C4, or (Cr,Fe,Ni)Σ9C4, Crystal system: Hexagonal, Space Group: P63/mc, a = 18.839(2) ?, c = 4.4960 (9) ?, V = 745.7(2) ?3, Z = 6, Density (calc.) = 7.19 g/cm3 (with simplified formula). Yarlongite has been approved as a new mineral by the CNMNC (IMA2007-035). Holotype material is deposited at the Geological Museum of China (No. M11650).