Biljana Lazic

Chegemite Ca7(SiO4)3(OH)2 – a new humite-group calcium mineral from the Northern Caucasus, Kabardino-Balkaria, Russia

The new mineral chegemite Ca7(SiO4)3(OH)2 ( Pbnm , Z = 4)1, a = 5.0696(1), b = 11.3955(1), c = 23.5571(3) A; V = 1360.91(4) A3 – the calcium and hydroxyl analogue of humite – was discovered as a rock-forming mineral in high-temperature skarns in calcareous xenoliths in ignimbrites of the Upper Chegem volcanic structure, Northern Caucasus, Kabardino-Balkaria, Russia. The chegemite forms granular aggregates with grain sizes up to 5 mm and is associated with various high-temperature minerals: larnite, spurrite, rondorfite, reinhardbraunsite, wadalite, lakargiite, and srebrodolskite, corresponding to the sanidinite metamorphic facies. The empirical formula of the holotype chegemite (mean of 68 analyses) is Ca7(Si0.997Ti0.003O4)3(OH)1.48F0.52. Chegemite is characterized by the following optical properties: 2VZ = −80(8)°, α = 1.621(2), β = 1.626(3), γ = 1.630(2); Δ = 0.009; density D calc = 2.892 g/cm3. The crystal structure, including hydrogen positions, has been refined from single-crystal Mo K α X-ray diffraction data to R = 2.2 %. Octahedral Ca–O distances are similar to those of γ-Ca2SiO4 (calcio-olivine). As is characteristic of OH-dominant humite-group minerals, two disordered H positions could be resolved. The main bands in the FTIR-spectra of chegemite are at 3550, 3542, 3475, 927, 906, 865, 820, 800, 756, 705, 653, 561, 519 and 437 cm−1. Those in non-polarized Raman spectra are at 389, 403, 526, 818, 923.5, 3478, 3551 and 3563 cm−1. The X-ray diffraction powder-pattern (Fe K α-radiation) shows the strongest lines {d \[A\]( I obs)} at: 1.907(10), 2.993(8), 2.700(8), 3.015(7), 2.720(7), 2.834(6), 3.639(5), and 3.040(5).

Vorlanite (CaU6+O4) - A new mineral from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia

Abstract The new mineral vorlanite, (CaU6+)O4, Dcalc = 7.29 g/cm3, H = 4-5, VHN10 = 360 kg/mm2, was found near the top of Mt. Vorlan in a calcareous skarn xenolith in ignimbrite of the Upper Chegem caldera in the Northern Caucasus, Kabardino-Balkaria, Russia. Vorlanite occurs as aggregates of black platy crystals up to 0.3 mm long with external symmetry 3̄m. The strongest powder diffraction lines are [d(Å)/(hkl)]: 3.107/(111), 2.691/(200), 1.903/(220), 1.623/(311), 1.235/(331), 1.203/(420), 1.098/(422), 0.910/(531). Single-crystal X-ray study gives isometric symmetry, space group Fm3̄m, a = 5.3813(2) Å, V = 155.834(10) Å3, and Z = 2. X-ray photoelectron spectroscopy indicate that all U in vorlanite is hexavalent. The mineral is isostructural with fluorite and uraninite (U4+O2). In contrast to synthetic rhombohedral CaUO4, and most U6+ minerals, the U6+ cations in vorlanite are present as disordered uranyl ions. [8]Ca2+ and [8]U6+ are disordered over a single site with average M-O = 2.33 Å. Vorlanite is believed to be a pseudomorphic replacement of originally rhombohedral CaUO4. We assume that this rhombohedral phase transformed by radiation damage to cubic CaUO4 (vorlanite). The new mineral is associated with larnite, chegemite, reinhardbraunsite, lakargiite, rondorfite, and wadalite, which are indicative of high-temperature formation (>800 °C) at shallow depth.

Elbrusite-(Zr)—A new uranian garnet from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia

Abstract Elbrusite-(Zr) Ca3(U6+Zr)(Fe3+2 Fe2+)O12, a new uranian garnet (Ia3̅d, a ≈ 12.55 Å, V ≈ 1977 Å3, Z = 8), within the complex solid solution elbrusite-kimzeyite-toturite Ca3(U,Zr,Sn,Ti,Sb,Sc,Nb...)2(Fe,Al,Si,Ti)3O12 was discovered in spurrite zones in skarn xenoliths of the Upper Chegem caldera. The empirical formula of holotype elbrusite-(Zr) with 25.14 wt% UO3 is (Ca3.040Th0.018Y0.001)Σ3.059(U6+0.658Zr1.040Sn0.230Hf0.009Mg0.004)Σ1.941(Fe3+1.575Fe2+0.559Al0.539Ti40.199Si0.099Sn0.025V5+0.004)Σ3O12. Associated minerals are spurrite, rondorfite, wadalite, kimzeyite, perovskite, lakargiite, ellestadite-(OH), hillebrandite, afwillite, hydrocalumite, ettringite group minerals, and hydrogrossular. Elbrusite-(Zr) forms grains up to 10-15 μm in size with dominant {110} and minor {211} forms. It often occurs as zones and spots within Fe3+-dominant kimzeyite crystals up to 20-30 μm in size. The mineral is dark-brown to black with a brown streak. The density calculated on the basis of the empirical formula is 4.801 g/cm3 The following broad bands are observed in the Raman spectra of elbrusite-(Zr): 730, 478, 273, 222, and 135 cm-1. Elbrusite-(Zr) is radioactive and nearly completely metamict. The calculated cumulative dose (α-decay events/mg) of the studied garnets varies from 2.50 × 1014 [is equivalent to 0.04 displacement per atom (dpa)] for uranian kimzeyite (3.36 wt% UO3), up to 2.05 × 1015 (0.40 dpa) for elbrusite-(Zr) with 27.09 wt% UO3.

Shulamitite Ca3TiFe3+AlO8 – a new perovskite-related mineral from Hatrurim Basin, Israel

Shulamitite, ideally Ca 3 TiFe 3+ AlO 8 , is a mineral intermediate between perovskite CaTiO 3 and brownmillerite Ca 2 (Fe,Al) 2 O 5 . It was discovered as a major mineral in a high-temperature larnite-mayenite rock from the Hatrurim Basin, Israel. Shulamitite is associated with larnite, F-rich mayenite, Cr-containing spinel, ye9elimite, fluorapatite, and magnesioferrite, and retrograde phases (portlandite, hematite, hillebrandite, afwillite, foshagite and katoite). The mineral forms reddish brown subhedral grains or prismatic platelets up to 200 μm and intergrowths up to 500 μm. The empirical formula of the holotype shulamitite (mean of 73 analyses) is (Ca 2.992 Sr 0.007 LREE 0.007 )(Ti 0.981 Zr 0.014 Nb 0.001 )(Fe 3+ 0.947 Mg 0.022 Cr 0.012 Fe 2+ 0.012 Mn 0.001 )(Al 0.658 Fe 3+ 0.288 Si 0.054 )O 8 . The X-ray diffraction powder-pattern (Mo Kα -radiation) shows the strongest lines {d [A]( I obs )} at: 2.677(100), 2.755(40), 1.940(40), 11.12(19), 1.585(17), 1.842(16), 1.559(16), 3.89 (13), 1.527(13). The unit-cell parameters and space group are: a = 5.4200(6), b = 11.064(1), c = 5.5383(7) A, V= 332.12(1) A 3 , Pmma, Z = 2. The calculated density is 3.84 g/cm 3 . The crystal structure of shulamitite has been refined from X-ray single-crystal data to R 1 = 0.029 %. No partitioning among octahedral sites was found for Ti and Fe 3+ in the structure of shulamitite, these cations are randomly distributed among all octahedra indicating an example of “valency-imposed double site occupancy”. The strong bands in the Raman spectrum of shulamitite are at: 238,250, 388,561, and 742 cm −1 . Shulamitite from the Hatrurim Basin crystallized under combustion metamorphism conditions characterized by very high temperatures (1150−1170 °C) and low pressures (high- T -region of the spurrite-merwinite facies). Chemical data for shulamitite and its Fe-analog from other metacarbonate occurrences (natural and anthropogenic) are given here.

Harmunite CaFe2O4: A new mineral from the Jabel Harmun, West Bank, Palestinian Autonomy, Israel

Abstract Harmunite, naturally occurring calcium ferrite CaFe2O4, was discovered in the Hatrurim Complex of pyrometamorphic larnite rocks close to the Jabel Harmun, the Judean Desert, West Bank, Palestinian Autonomy, Israel. The new mineral occurs in larnite pebbles of the pseudo-conglomerate, the cement of which consists of intensely altered larnite-bearing rocks. Srebrodolskite, magnesioferrite, and harmunite are intergrown forming black porous aggregates to the central part of the pebbles. Larnite, fluorellestadite, ye’elimite, fluormayenite, gehlenite, ternesite, and calciolangbeinite are the main associated minerals. Empirical crystal chemical formula of harmunite from type specimen is as follows Ca1.013(Fe3+ 1.957Al0.015Cr3+ 0.011Ti4+0.004 Mg0.003)S1.993O4. Calculated density is 4.404 g/cm3, microhardness VHN50 is 655 kg/mm2. The Raman spectrum of harmunite is similar to that of the synthetic analog. Harmunite in hand specimen is black and under reflected plane-polarized light is light gray with red internal reflections. Reflectance data for the COM wavelengths vary from ~22% (400 nm) to ~18% (700 nm). The crystal structure of harmunite [Pnma; a = 9.2183(3), b = 3.0175(1), c = 10.6934(4) Å; Z = 4, V = 297.45(2) Å3], analogous to the synthetic counterpart, was refined from X-ray single-crystal data to R1 = 0.0262. The structure of CaFe2O4 consist of two symmetrically independent FeO6 octahedra connected over common edges, forming double rutile-type ∞1[Fe2O6] chains. Four such double chains are further linked by common oxygen corners creating a tunnel-structure with large trigonal prismatic cavities occupied by Ca along [001]. The strongest diffraction lines are as follows [dhkl, (I)]: 2.6632 (100), 2.5244 (60), 2.6697 (52), 1.8335 (40), 2.5225 (35), 2.2318 (34), 1.8307 (27), 1.5098 (19). Crystallization of harmunite takes place in the presence of sulfate melt. ̃

Bitikleite-(SnAl) and bitikleite-(ZrFe): New garnets from xenoliths of the Upper Chegem volcanic structure, Kabardino-Balkaria, Northern Caucasus, Russia

Abstract Two new antimonian garnets-bitikleite-(SnAl) Ca3SbSnAl3O12 and bitikleite-(ZrFe) Ca3SbZrFe3O12-have been found as accessory minerals in the cuspidine zone of high-temperature skarns in a carbonate-silicate xenolith at the contact with ignimbrites within the Upper Chegem structure in the Northern Caucasus, Kabardino-Balkaria, Russia. The bitikleite series forms a solid solution with garnets of the kimzeyite-schorlomite and toturite type. Antimony-garnets form crystals up to 50 μm across containing kimzeyite cores and thin subsequent zones of complex lakargiite-tazheranitekimzeyite pseudomorphs after zircon. Bitikleite-(SnAl) has a = 12.5240(2) Å, V = 1964.40(3) Å3 and bitikleite-(ZrFe) has a = 12.49 Å, V = 1948.4 Å3 (Ia3d, Z = 8). The strongest powder diffraction lines of bitikleite-(SnAl) are [d, Å (hkl)]: 4.407 (220), 3.118 (440), 2.789 (420), 2.546 (422), 1.973 (620), 1.732 (640), 1.668 (642), and 1.396 (840). The strongest calculated powder diffraction lines of bitikleite-(ZrFe) are [d, Å (hkl)]: 4.416 (220), 3.123 (440), 2.793 (420), 2.550 (422), 1.975 (620), 1.732 (640), 1.669 (642), and 1.396 (840). The Raman spectra of bitikleite garnets are similar to the spectra of kimzeyite and toturite. Larnite, rondorfite, wadalite, magnesioferrite, tazheranite, lakargiite, kimzeyite, and toturite associated with bitikleite garnets are typical of high-temperature (>800 °C) formation

Thermodynamic and crystallographic properties of kornelite [Fe-2(SO4)(3)center dot similar to 7.75H(2)O] and paracoquimbite [Fe-2(SO4)(3)center dot 9H(2)O]

Abstract Enthalpies of formation of kornelite [Fe2(SO4)3·~7.75H2O] and paracoquimbite [Fe2(SO4)3·9H2O] were measured by acid (5 N HCl) solution calorimetry at T = 298.15 K. The samples were characterized chemically by an electron microprobe, and structurally by the means of single-crystal, in-house powder, and synchrotron powder X-ray diffraction. The refined structures for the two phases are provided, including estimates of the positions and concentration of non-stoichiometric water in structural channels of kornelite, location of the hydrogen atoms and the hydrogen bonding system in this phase. The measured enthalpies of formation from the elements (crystalline Fe, orthorhombic S, ideal gases O2 and H2) at T = 298.15 K are -4916.2 ± 4.2 kJ/mol for kornelite and -5295.4 ± 4.2 kJ/mol for paracoquimbite. We have used several algorithms to estimate the standard entropy of the two phases. Afterward, we calculated their Gibbs free energy of formation and constructed a phase diagram for kornelite, paracoquimbite, Fe2(SO4)3·5H2O, and Fe2(SO4)3 as a function of temperature and relative humidity of air. The topology of the phase diagram is very sensitive to the entropy estimates and the construction of a reliable phase diagram must await better constraints on entropy or Gibbs free energy of formation. Possible remedies of these problems are also discussed

Thermodynamic and crystallographic properties of kornelite [Fe2(SO4)3·~7.75H2O] and paracoquimbite [Fe2(SO4)3·9H2O]

Abstract Enthalpies of formation of kornelite [Fe2(SO4)3·~7.75H2O] and paracoquimbite [Fe2(SO4)3·9H2O] were measured by acid (5 N HCl) solution calorimetry at T = 298.15 K. The samples were characterized chemically by an electron microprobe, and structurally by the means of single-crystal, in-house powder, and synchrotron powder X-ray diffraction. The refined structures for the two phases are provided, including estimates of the positions and concentration of non-stoichiometric water in structural channels of kornelite, location of the hydrogen atoms and the hydrogen bonding system in this phase. The measured enthalpies of formation from the elements (crystalline Fe, orthorhombic S, ideal gases O2 and H2) at T = 298.15 K are -4916.2 ± 4.2 kJ/mol for kornelite and -5295.4 ± 4.2 kJ/mol for paracoquimbite. We have used several algorithms to estimate the standard entropy of the two phases. Afterward, we calculated their Gibbs free energy of formation and constructed a phase diagram for kornelite, paracoquimbite, Fe2(SO4)3·5H2O, and Fe2(SO4)3 as a function of temperature and relative humidity of air. The topology of the phase diagram is very sensitive to the entropy estimates and the construction of a reliable phase diagram must await better constraints on entropy or Gibbs free energy of formation. Possible remedies of these problems are also discussed

Kumtyubeite Ca5(SiO4)2F2—A new calcium mineral of the humite group from Northern Caucasus, Kabardino-Balkaria, Russia

Abstract Kumtyubeite, Ca5(SiO4)2F2-the fluorine analog of reinhardbraunsite with a chondrodite-type structure-is a rock-forming mineral found in skarn carbonate-xenoliths in ignimbrites of the Upper Chegem volcanic structure, Kabardino-Balkaria, Northern Caucasus, Russia. The new mineral occurs in spurrite-rondorfite-ellestadite zones of skarn. The empirical formula of kumtyubeite from the holotype sample is Ca5(Si1.99Ti0.01)Σ2O8(F1.39OH0.61)Σ2. Single-crystal X-ray data were collected for a grain of Ca5(SiO4)2(F1.3OH0.7) composition, and the structure refinement, including a partially occupied H position, converged to R = 1.56%: monoclinic, space group P21/a, Z = 2, a = 11.44637(18), b = 5.05135(8), c = 8.85234(13) Å, β = 108.8625(7)°, V = 484.352(13) Å3. For direct comparison, the structure of reinhardbraunsite Ca5(SiO4)2(OH1.3F0.7) from the same locality has also been refined to R = 1.9%, and both symmetry independent, partially occupied H sites were determined: space group P21/a, Z = 2, a = 11.4542(2), b = 5.06180(10), c = 8.89170(10) Å, β = 108.7698(9)°, V = 488.114(14) Å3. The following main absorption bands were observed in kumtyubeite FTIR spectra (cm-1): 427, 507, 530, 561, 638, 779, 865, 934, 1113, and 3551. Raman spectra are characterized by the following strong bands (cm-1) at: 281, 323, 397 (ν2), 547 (ν4), 822 (ν1), 849 (ν1), 901 (ν3), 925 (ν3), 3553 (VOH). Kumtyubeite with compositions between Ca5(SiO4)2F2 and Ca5(SiO4)2(OH1.0F1.0) has only the hydrogen bond O5-H1···F5′, whereas reinhardbraunsite with compositions between Ca5(SiO4)2(OH1.0F1.0) and Ca5(SiO4)2(OH)2 has the following hydrogen bonds: O5-H1···F5′, O5-H1···O5′, and O5-H2···O2.

Rusinovite, Ca10(Si2O7)3Cl2: a new skarn mineral from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia

Rusinovite, Ca 10 (Si 2 O 7 ) 3 Cl 2 , was discovered in an altered carbonate-silicate xenolith enclosed in ignimbrites of the Upper Chegem volcanic caldera. The mineral is named after Vladimir Leonidovich Rusinov (1935–2007), a Russian petrologist and expert in the field of thermodynamics of non-equilibrium mineral systems. A synthetic analogue of rusinovite is also known. The new mineral has an OD structure of which only the average structure could be determined based on strong and sharp reflections recorded by single-crystal X-ray diffraction: space group Cmcm , a = 3.7617(2), b = 16.9385(8), c = 17.3196(9) A, V = 1103.56(10) A 3 , Z = 2. The average structure ( R 1 = 3.18 %) is characterized by columns of face-sharing disilicate units extending parallel to a . However, in the true structure only each second (Si 2 O 7 ) unit is occupied. Although rusinovite has a stoichiometry similar to the apatite-group mineral nasonite, Pb 6 Ca 4 (Si 2 O 7 ) 3 Cl 2 , the two structures are considerably different. Rusinovite has following optical properties: α = 1.645(2), β = 1.664(2), γ = 1.675(3); Δ = 0.030, 2V meas = −75(10) °; 2V calc = −74.6 °; the Mohs hardness is 4–5, the density is 2.91 g/cm 3 . The mineral forms fibrous crystals often intergrown into spherolites and displays good cleavage parallel to (010). The Raman spectrum of rusinovite strongly resembles that of another skarn calcium-disilicate: rankinite, Ca 3 Si 2 O 7 .