O. V. Karimova

Modulation in the murataite structure

Synthetic varieties of murataites (M5 and M8) were studied by transmission electron microscopy. One of the varieties was additionally investigated by high-resolution electron microscopy. It was demonstrated that the atoms possessing the strongest scattering properties are nonuniformly distributed over the unit cells of both varieties, and the distribution follows the laws of replacement and displacement modulations of cations in the ideal fluorite structure.

Geology, mineralogy, and genesis of PGE mineralization in the South Sopcha massif, Monchegorsk complex, Russia

New data are reported on the localization and genesis of PGE mineralization at the South Sopcha deposit situated in the southern framework of the Monchegorsk pluton. Disseminated PGE-Cu-Ni mineralization, the thickness of which in particular boreholes exceeds 100 m, is hosted in the zone of alternating peridotite, pyroxenite, norite, and gabbronorite. The PGE grade does not exceed 1–2 gpt with Pd/Pt = 3–4 at Ni and Cu contents from 0.2 to 1.5 wt %. The PGE contents up to 4–6 gpt and Pd/Pt = 4–8 are noted at local sites of hydrothermally altered rocks. Another type of PGE mineralization is established in the outcrops of the southeastern marginal group of the massif. Pyroxenite, norite, and gabbronorite fragments are incorporated here in the gabbroic matrix, making up a complex zone of magmatic breccia complicated by mylonites and late injections. Elevated PGE contents (1.0–6.5 gpt) are detected in all types of rocks in the zone of brecciation, mainly in the matrix. Platinum-group minerals (PGM) occur in association with magmatic and late sulfides, amphibole, mica, and chlorite. PGM vary in composition depending on the petrographic features of rocks. In rocks of the layered series and in pegmatoid pyroxenite PGM are extremely diverse comprising PGE compounds with As, Sb, Bi, Te, Se, and S. In the brecciated rocks of the marginal group, Pd bismuthotellurides (mainly merenskyite), sperrylite, hollingworthite, and Pd- and Rh-bearing cobaltite and gersdorffite are predominant. The PGE mineralization in rocks of the layered series and pegmatoid pyroxenite was formed from the magmatic melt enriched in volatiles and with subsequent transformation of PGE assemblages under the influence of hydrothermal fluids at a lower temperature. In gabbroic rocks of the marginal group, PGM are associated with the latest sulfides (chalcopyrite, bornite, chalcocite), forming separate grains and thin veinlets in hydrothermally altered rocks. The gabbroic melt affected incompletely crystallized rocks of the layered series by formation of contact-type PGE mineralization, deposition and redeposition of ore matter.

Ordering of cations in the voids of the anionic framework of the crystal structure of catapleiite

The pseudohexagonal crystal structure of the mineral catapleiite Na1.5Ca0.2[ZrSi3(O,OH)9] · 2(H2O,F) from the Zhil’naya Valley in the central part of the Khibiny alkaline massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0346): a = 20.100(4), b = 25.673(5), and c = 14.822(3) Å; space group Fdd2, Z = 32, and ρcalcd = 2.76 g/cm3. Fluorine atoms substituting part of H2O molecules in open channels of the crystal structure have been found for the first time in the catapleiite composition by microprobe analysis. The pattern of distribution of Na and Ca atoms over the voids of the mixed anionic framework consisting of Zr-octahedra and three-membered rings of Si-tetrahedra accounts for the pronounced pseudoperiodicity along the a and c axes of the pseudohexagonal unit cell and for the lowering of crystal symmetry to the orthorhombic one. It is shown that part of the hydrogen atoms of water molecules is statistically disordered; their distribution correlates with the pattern of the population of large eight-vertex polyhedra by Na and Ca atoms.

The crystal structure of arangasite, Al2F(PO4)(SO4)·9H2O determined using low-temperature synchrotron data

Abstract The crystal structure of the fibrous mineral arangasite, Al2F(PO4)(SO4)·9H2O from the Alyaskitovoje deposit, Eastern Yakutiya, Russia, was solved using low-temperature single-crystal data from synchrotron radiation and refined against F2 to R = 9.8%. Arangasite crystallizes in the monoclinic space group P2/a, with unit-cell parameters a = 7.073(1), b = 9.634(2), c = 10.827(2) Å, β = 100.40(1)º, V = 725.7(7) Å3 and Z = 2. The positions of all the independent H atoms were obtained by difference- Fourier techniques and refined in an isotropic approximation. The arangasite crystal structure is built from one-dimensional chains of Al octahedra and PO4 tetrahedra sharing vertices, quasi-isolated SO4 tetrahedra and H2O molecules. All O atoms are involved in the system of H bonding, acting as donors and/or acceptors. Hydrogen bonding serves as the only mechanism providing linkage between the main structural fragments, thus maintaining the framework. Chains of corner-sharing Al octahedra and P tetrahedra in the arangasite structure are topologically identical to the chains built from (Fe, Al) octahedra and P tetrahedra in the crystal structure of destinezite, Fe2(OH)(PO4)(SO4)·6H2O. It has been shown that in spite of very similar chemical formulae, arangasite and sanjuanite, Al2(OH)(PO4)(SO4)·9H2O, are not isotypic.

Oxyphlogopite K(Mg,Ti,Fe)3[(Si,Al)4O10](O,F)2: A new mineral species of the mica group

Oxyphlogopite is a new mica-group mineral with the idealized formula K(Mg,Ti,Fe)3[(Si,Al)4O10](O,F)2. The holotype material came from a basalt quarry at Mount Rothenberg near Mendig at the Eifel volcanic complex in Rhineland-Palatinate, Germany. The mineral occurs as crystals up to 4 × 4 × 0.2 mm in size encrusting cavity walls in alkali basalt. The associated minerals are nepheline, plagioclase, sanidine, augite, diopside, and magnetite. Its color is dark brown, its streak is brown, and its luster is vitreous. Dmeas = 3.06(1) g/cm3 (flotation in heavy liquids), and Dcalc = 3.086 g/cm3. The IR spectrun does not contain bands of OH groups. Oxyphlogopite is biaxial (negative); α = 1.625(3), β = 1.668(1), and γ = 1.669(1); and 2Vmeas = 16(2)° and 2Vcalc = 17°. The dispersion is strong; r < ν. The pleochroism is medium; X > Y > Z (brown to dark brown). The chemical composition is as follows (electron microprobe, mean of 5 point analyses, wt %; the ranges are given in parentheses; the H2O was determined using the Alimarin method; the Fe2+/Fe3+ was determined with X-ray emission spectroscopy): Na2O 0.99 (0.89–1.12), K2O 7.52 (7.44–7.58), MgO 14.65 (14.48–14.80), CaO 0.27 ((0.17–0.51), FeO 4.73, Fe2O3 7.25 (the range of the total iron in the form of FeO is 11.09–11.38), Al2O3 14.32 (14.06–14.64), Cr2O3 0.60 (0.45–0.69), SiO2 34.41 (34.03–34.66), TiO2 12.93 (12.69–13.13), F 3.06 (2.59–3.44), H2O 0.14; O=F2 −1.29; 99/58 in total. The empirical formula is (K0.72Na0.14Ca0.02)(Mg1.64Ti0.73Fe0.302+ Fe0.273+Cr0.04)Σ2.98(Si2.59Al1.27Fe0.143+ O10) O1.20F0.73(OH)0.07. The crystal structure was refined on a single crystal. Oxyphlogopite is monoclinic with space group C2/m; the unit-cell parameters are as follows: a = 5.3165(1), b = 9.2000(2), c = 10.0602(2) Å, β = 100.354(2)°. The presence of Ti results in the strong distortion of octahedron M(2). The strongest lines of the X-ray powder diffraction pattern [d, Å (I, %) [hkl]] are as follows: 9.91(32) [001], 4.53(11) 110], 3.300(100) [003], 3.090(12) [112], 1.895(21) [005], 1.659(12) [−135], 1.527(16) [−206, 060]. The type specimens of oxyphlogopite are deposited at the Fersman Mineralogical Museum in Moscow, Russia; the registration numbers are 3884/2 (holotype) and 3884/1 (cotype).

Ti/Zr isomorphism in wadeite: The crystal structure of the titanium-dominant K2(Ti0.55Zr0.45)Si3O9 member of the series

The crystal structure of the titanium-rich mineral wadeite K2(Ti0.55Zr0.45)Si3O9 from rischorrites of the Khibiny Alkaline Massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0459): a = 6.8611(6) Å and c = 10.0611(9) Å; space group P63/m, Z = 6, Dx = 3.03 g/cm3. It is shown that the unit-cell parameters and volume of the mineral of mixed (Ti/Zr) composition are naturally intermediate between those of the terminal members of the isomorphous wadeite-based K2ZrSi3O9–K2(Ti0.55Zr0.45)Si3O9–K2TiSi3O9 series. The expected correlation is due to the ionic radii of Zr4+ and Ti4+ which determine the lengths of Zr/Ti–O bonds in octahedra. The data of field observations and microscopic studies show that the Ti-dominant wadeite is formed on the basis of primary zirconium mineral in the course of a late imposed process under unique geochemical conditions.

Crystal structure interconnections in a family of hydrated phosphate-sulfates

The phosphate-sulfate family incorporates several water-containing hypergene minerals with various structures. We determined the crystal structure of lately discovered [1] fibrous mineral arangasite, [Al2F(H2O)6(PO4)(SO4)]•3(H2O) using single-crystal synchrotron diffraction at 100 K (a =7.073(1), b=9.634(2), c=10.827(2) Å, β=79.60(1)°, P2/a, Z=2). Its crystal chemical interpretation has allowed us to reveal some interesting features in a title group of compounds. The arangasite crystal structure is dominated by chains extending in the [100] direction and built of pairs of corner-shared Al octahedra joined through bridging F atoms and P tetrahedra. They alternate in the [001] with S tetrahedra forming layers parallel to the ac plane through a system of hydrogen bonds. Along [010] the complex layers are separated by layers of H2O molecules. Hydrogen bonding serves here as the only mechanism providing linkage between the main structural fragments. The Al/ P chains are topologically identical to the chains built from Fe octahedra and P tetrahedra in the triclinic structure of destinezite, Fe2(OH)(PO4)(SO4)(H2O)6 [2]. The repeating subunit of both chains consists of two octahedra and one tetrahedron sharing vertices. A main difference among the chains arises from their chemistry; Al octahedra in arangasite form pairs by sharing the F vertex of neighboring polyhedra, whereas pairs of Fe octahedra in destinezite are linked together through the oxygen vertex of an OH group. As a result, the larger size of the Fe octahedra compared to Al octahedra causes a larger c = 7.31 Å along the chain in destinezite. Additional SO4 tetrahedra here are attached to these chains along their periphery through an oxygen vertex bridge with Fe octahedra. The monoclinic sanjuanite, Al2(PO4)(SO4)(OH)(H2O)9 structure [3] is composed of Al/P chains, parallel to a = 6.11 Å. These chains are also built from three-member units that include corner-sharing pairs of octahedra connected by PO4 tetrahedron, but they are not topologically equivalent to the chains in the arangasite and destinezite structures. Similar to arangasite, sulfate groups and H2O molecules reside between chains in the sanjuanite structure with hydrogen bonding. Thus, similar the crystal chemical formulae of sanjuanite and arangasite differ with respect to the (OH) → F substitution, which results in contrasting unit cell parameters. Note, that the unit cell volume of sanjuanite, is twice as large as arangasite.

Crystal Structure of Magnesioneptunite

The crystal structure of the new mineral magnesioneptunite (K0.8Na0.1−0.1)Na2Li(Ti0.39Mg0.34Fe0.27)2(Ti0.59Mg0.22Fe0.19)2[Si4O11]2(O,OH) from the xenolith of Verkhnechegemskaya caldera (Lakarga Mountain, North Caucasus) has been investigated by X-ray diffraction (XCalibur-S diffractometer, R = 0.0244): a = 16.3271(7) Å, b = 12.4788(4) Å, c = 9.9666(4) Å β = 115.651(5), V = 1830.5(1) Å3, sp. gr. C2/c, Z = 4, and ρcalcd = 3.152 g/cm3. The disordered distribution of Ti, Mg, and Fe atoms in the octahedra forming the basis of the cationic framework is established. It is shown that the isomorphic occupation of octahedral positions by cations of three types corresponds to the centrosymmetric crystal structure and is likely caused by the high-temperature crystallization of the mineral.