Nine samples of carbonate-free sodalite-group minerals, including those with abnormally high contents of polysulfide groups, fluoride anion and carbon dioxide molecules as well as synthetic fluoraluminate sodalite-type compound Na8(Si7Al5O24)(AlF6)3–·5H2O, have been studied by means of electron microprobe analyses, infrared and Raman spectroscopy; the CO2 content was determined using the selective sorption of gaseous ignition products. This article describes a semi-quantitative method for estimating the content of carbon dioxide molecules in these minerals, based on IR spectroscopy data. The data obtained demonstrate the existence of a sulfide sodalite-group mineral with the idealized formula Na7(Si6Al6O24)(S3−)·H2O, which differs significantly from the formula Na6Ca2(Si6Al6O24)S2–2 accepted for lazurite. According to single-crystal X-ray structural analysis, in the F-rich sodalite-group mineral from the Eifel paleovolcanic region, Germany with the idealized formula Na7(Si6Al6O24)F−·nH2O fluorine occurs as an isolated F− anion, unlike synthetic F-rich sodalite-type compounds.
Elpidite from the Lovozero alkaline complex, Kola Peninsula, Russia, and Ag-exchanged forms of elpidite from two different localities (Lovozero and Khan Bogdo, Mongolia) were studied by means of single-crystal X-ray diffraction, electron microprobe analysis, thermogravimetry and IR spectroscopy. All studied samples retain the heteropolyhedral framework consisting of double Si6O15 chains (ribbons) and isolated ZrO6 octahedra. Zeolitic cavities in the initial elpidite from Lovozero (space group Pbm2, a = 14.6127(7), b = 7.3383(4), c = 7.1148(3) Å, V = 762.94(6) Å3) are occupied by Na+ cations and H2O molecules. Both Ag-exchanged forms are characterized by evident distortions of the heteropolyhedral framework and a strongly disordered arrangement of extra-framework cations which results in the appearance of the 14-14-14 Å unit cell (a = 14.1755(7), b = 14.6306(9), c = 14.2896(7) Å, V = 2963.6(3) Å3 for the Ag-exchanged form of elpidite from Lovozero and a = 14.1411(5), b = 14.5948(4), c = 14.3035(5) Å, V = 2952.04(17) Å3 for the Ag-exchanged form of elpidite from Khan Bogdo) and space group Cmce. Elpidite from both localities demonstrates a high exchange capacity to Ag. Exchanged Ag+ cations preferably occupy the sites that are close to the Na sites in the initial elpidite. The paper also contains a review of crystal chemical data on elpidite and its laboratory-modified forms.
The new mineral fluorcalciobritholite, ideally Ca 3 Ce 2 (SiO 4) 2 (PO 4)F, has been found at Mount Kukisvumchorr, Khibiny alkaline complex, Kola Peninsula, Russia, in veinlets which contains aggregates of orthoclase, nepheline, sodalite and biotite in association with grains of fayalite, gadolinite-(Ce), zircon, monazite-(Ce), zirconolite ("polymignite"), fluorapatite, fluorite, molybdenite, löllingite and graphite. Fluorcalciobritholite forms long-prismatic hexagonal crystals up to 0.5 x 10 mm; the main crystal form is the hexagonal prism {10-10}. The mineral is transparent, with a pale pinkish to brown colour and a white streak. The hardness (Mohs) is 5.5, and the observed density is 4.2(1) g/cm 3. Optically, it is uniaxial (-) with K 1.735(5), 5 1.730(5). Electron microprobe gave the following empirical formula based on [Si+P+S] = 3 apfu: [Ca 2.80 (Ce 0.93 La 0.54 Nd 0.26 Y 0.18 Pr 0.08 Sm 0.03 Gd 0.03 Dy 0.02 Yb 0.02 Er 0.01) 7 2.12 Th 0.04 Mn 0.03 Sr 0.02 ] 7 4.99 [(Si 1.94 P 1.06) 7 3 O 12 ] [F 0.76 O 0.22 Cl 0.01 ] 7 0.99 (Z = 2). The IR spectrum of metamict fluorcalciobritholite from Siberia showed a marked similarity with those of hydroxylbritholite-(Ce) and hydroxylbritholite-(Y). The strongest lines of the X-ray powder pattern [d in Å (I) (hkl)] are: 3.51 (45) 002, 3.15 (70) 102, 2.85 (100) 211, 121, 2.78 (60) 300. The mineral is hexagonal, space group P6 3 /m, with a = 9.580(7), c = 6.985(4) Å, V = 555.2(7) Å 3. The crystal structure was refined from single-crystal X-ray diffraction data to R F = 0.029. Fluorcalciobritholite, whose simplified formula is (Ca,REE) 5 [(Si,P)O 4 ] 3 F, differs from fluorbritholite in having Ca 8 7 REE, and differs from fluorapatite in having Si 8 P. Its compositional field falls within the limits Ca 2.5 REE 2.5 (SiO 4) 2.5 (PO 4) 0.5 F (boundary with fluorbritholite) and Ca 3.5 REE 1.5 (SiO 4) 1.5 (PO 4) 1.5 F (boundary with fluorapatite). Both the mineral and its name have been approved by the IMA Commission on New Minerals and Mineral Names.
The crystal-chemical characterization of oxysalts (sulfates, arsenates, vanadates, selenites, silicates, molybdates and borates), chlorides and oxides with speciesdefining Cu 2+ formed in volcanic fumaroles (96 minerals representing 80 structure types; 81 species are endemic to fumarolic formation) is given. Copper minerals are known only from oxidizing-type fumaroles. The most diverse copper mineralization occurs at the Tolbachik volcano (Kamchatka, Russia). Copper minerals from fumarolic systems are subdivided into two genetic groups: Group I are minerals formed in the hot zones of fumaroles (>473 K, mainly 673-973 K) and Group II are minerals formed in the moderately hot zones of fumaroles (<473 K, mainly at 343-423 K). Group I includes 81 mineral species. Their most defining chemical feature is that all of them are hydrogen-free, and many of them contain the additional anion O 2À . In comparison with minerals from other geological environments, in minerals of Group I the Cu 2+ cation exhibits the strongest affinity for four-and fivefold coordinations and the strongest distortion of Cu 2+ -centred octahedra. Group II consists of 15 chlorides and sulfates including 13 H-bearing species. In these minerals the Cu 2+ cation shows affinity for octahedral coordination, with OH À and/or H 2 O 0 as ligands. In terms of crystal chemistry these minerals are closer to supergene minerals rather than to high-temperature fumarolic species. Temperature is the major factor governing the crystal chemistry of Cu 2+ oxysalts and chlorides in low-pressure systems. The defining feature of fumarolic copper mineralization over this whole temperature range is the important role of alkali cations. The available data on complexes of Cu 2+ -centred polyhedra in the structures of natural oxysalts and halides are summarized and reviewed. Isomorphism in copper minerals from volcanic exhalations is discussed. The structures of high-temperature Cu oxysalts with additional O 2À anions (i.e. O atoms non-bonded to S 6+ , Mo 6+ , As 5+ , V 5+ , Se 4+ or B 3+ ) are also interpreted using an approach based on oxocentred tetrahedra. mineralogical crystallography Acta Cryst. (2018). B74, 502-518 Igor V. Pekov et al. Copper minerals from volcanic exhalations 503 mineralogical crystallography 504 Igor V. Pekov et al. Copper minerals from volcanic exhalations Acta Cryst. (2018). B74, 502-518
Two new minerals, ericlaxmanite and kozyrevskite, dimorphs of Cu4O(AsO4)2, were found in sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. They are associated with each other and with urusovite, lammerite, lammerite-b, popovite, alarsite, tenorite, hematite, aphthitalite, langbeinite, As-bearing orthoclase, etc. Ericlaxmanite occurs as tabular, lamellar, equant or short prismatic crystals up to 0.1 mm in size, their clusters and pseudomorphs after urusovite crystal crusts up to 1.5 cm × 2 cm in area. Kozyrevskite occurs as prismatic crystals up to 0.3 mm long in clusters and as individual crystals. Both minerals are transparent with a vitreous lustre. They are brittle, with Mohs’ hardness ~3–. Ericlaxmanite is green to dark green. Kozyrevskite is bright grass green to light yellowish green; Dcalc is 5.036 (ericlaxmanite) and 4.934 (kozyrevskite) g cm–3. Both minerals are optically biaxial (–); ericlaxmanite: α = 1.870(10), β = 1.900(10), γ = 1.915(10), 2Vmeas = 60(15)º; kozyrevskite: α = 1.885(8), β = 1.895(8), γ = 1.900(8), 2Vmeas. = 75(10)º. The Raman spectra are given. Chemical data (wt.%, electron microprobe; the first value is for ericlaxmanite, the second for kozyrevskite): CuO 57.55, 58.06; ZnO 0.90, 1.04; Fe2O3 0.26, 0.12; SiO2 n.d., 0.12; P2O5 0.23, 1.23; V2O5 0.14, 0.37; As2O5 40.57, 38.78; SO3 0.17, 0.43; total 99.82, 100.15. The empirical formulae, based on 9 O a.p.f.u., are: ericlaxmanite: (Cu3.97Zn0.06Fe0.02)Σ4.05(As1.94P0.02V0.01S0.01)Σ1.98O9 and kozyrevskite: (Cu3.95Zn0.07Fe0.01)Σ4.03(As1.83P0.09S0.03V0.02Si0.01)Σ1.98O9. Ericlaxmanite is triclinic, P1̄ , a = 6.4271(4), b = 7.6585(4), c = 8.2249(3) Å , α = 98.396(4), β = 112.420(5), γ = 98.397(5)º, V = 361.11(3) Å3 and Z = 2. Kozyrevskite is orthorhombic, Pnma, a = 8.2581(4), b = 6.4026(4), c = 13.8047(12) Å , V = 729.90(9) Å3 and Z = 4. The strongest reflections in the X-ray powder patterns [d Å (I)(hkl)] are: ericlaxmanite: 3.868(46)(101), 3.685(100)(020), 3.063(71)(012), 2.957(58)(02̄ 2), 2.777(98)(2̄ 12, 2̄ 1̄ 1), 2.698(46)(2̄1̄ 2) and 2.201(51)(013, 031); kozyrevskite: 3.455(100)(004), 3.194(72)(020, 104), 2.910(69)(022), 2.732(82)(122), 2.712(87)(301) and 2.509(92)(123). Their crystal structures, solved from single-crystal X-ray diffraction data [R = 0.0358 (ericlaxmanite) and 0.1049 (kozyrevskite)], are quite different. The ericlaxmanite structure is based on an interrupted framework built by edge- and corner-sharing Cu-centred, distorted tetragonal pyramids, trigonal bipyramids and octahedra. The kozyrevskite structure is based on complicated ribbons of Cu-centred distorted tetragonal pyramids and trigonal bipyramids. Ericlaxmanite is named in honour of the Russian mineralogist, geologist, geographer, biologist and chemist Eric Laxman (1737–1796). Kozyrevskite is named in honour of the Russian geographer, traveller and military man Ivan Petrovich Kozyrevskiy (1680–1734), one of the first researchers of Kamchatka.
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