The crystal structure of sturmanite

Sturmanite, ideally Ca6Fe3+2(SO4)2.5[B(OH)4](OH)12•25H2O, is trigonal, P31c, Z = 2, with a 11.188(9), c 21.91(7) Å, V 2375.1(5) Å3. The crystal structure has been refined to an R index of 0.079. As in the case of ettringite, two coaxial elements parallel to [001] can be considered as the most distinctive features of sturmanite. The main one is a column of polyhedra formed by Fe-octahedra and Ca-polyhedra. The Fe(OH)6 octahedra are located along the three-fold axes that pass through the origin of the cell. They are linked to three VIIICa-polyhedra, located around the three-fold axes, in between the Fe(OH)6 octahedra. The second structural element parallel to [001] contains [SO4] and B(OH)4 tetrahedra; it is located along three-fold axes at (⅓, ⅔, z) and (⅔, ⅓, z). Both structural modules are linked to each other by a complex system of hydrogen bonds. On the basis of a complete determination of the structure, the chemical composition of sturmanite is close to Ca6[Fe3+0.6Al0.2Mn4+0.2]2[SO4]2.7[B(OH)3]0.3 [B(OH)4](OH)12•24H2O

Larisaite, Na(H3O)(UO2)3(SeO3)2O2.4H2O, a new uranyl selenite mineral from Repete mine, San Juan County, Utah, U.S.A

Larisaite, a new uranyl selenite with the idealized formula Na(H3O)(UO2)3(SeO3)2O2 ·4H2O, has been found in a sedimentary rock from Repete mine near Blanding, San Juan Co., Utah, U.S.A., in association with quartz, haynesite, andersonite, wölsendorfite, uranophane, gypsum, calcite and montmorillonite. The mineral is named in memory of Russian mineralogist and crystallographer Larisa Nikolaevna Belova (1923-1998) who made a significant contribution to the knowledge on the uranium minerals. Larisaite forms coarse lamellar crystals up to 1 mm and radial aggregates up to 2 mm. It is transparent or translucent, yellow, lustre vitreous, streak yellow. Fluorescence under the UV light is green (wavelengths of excitation 250 nm). Larisaite is sectile, with Mohs’ hardness 1, perfect cleavage on (010) and uneven fracture across the cleavage direction. Calculated density is 4.50 g/cm3 from the crystal structure refinement and 4.46 g/cm3 from the empirical formula. Optically biaxial (-), alpha 1.597(2), beta 1.770(5), gamma 1.775(5); 2V = 20°. Dispersion is strong, r alpha (light greenish-yellow). IR spectrum is given. Average values for 3 point microprobe analyses (wt.%, ranges are given in brackets) are: Na2O 2.04 (1.82-2.32), K2O 0.69 (0.62-0.76), CaO 0.23 (0.17-0.30), UO3 72.19 (71.77-72.64), SeO2 18.12 (17.83-18.48); H2O content determined by Penfield method is 7.64; total 100.91 wt.%; contents of Mg, Sr, Ba, Pb, Zn, Mn, Ni, Co, Cu, Fe, Al, Si, S, As, Cl, F are lower than detection limits i. e. < 0.02-0.05%. The empirical formula based on (SeO3)2O2 is: Na0.81K0.18Ca0.05(H3O)0.73(UO2)3.09(SeO3)2O2·4.1H2O. 1-(KP/Kc) = 0.013 (“superior”). The crystal structure has been determined (R=0.067). Larisaite is monoclinic, space group P11m; a = 6.9806(9), b = 7.646(1), c = 17.249(2) Å, gamma = 90.039(4)°, V = 920.64 Å3, Z = 2. The strongest lines in the powder diffraction pattern [d, Å (I,%) (hkl)] are: 8.63 (43) (002), 7.67 (100) (010), 3.85 (40) (-113, 020, 113), 3.107 (77) (211), 2.874 (53) (006, -115). By the U : Se ratio, the values of unit cell parameters and the structure type, larisaite is related to haynesite, guilleminite and piretite. In common with guilleminite, uranium polyhedra and SeO3 triangles form the sheets, however the distribution of interlayer cations and H2O molecules is different. Holotype specimen is deposited in the Geoscientific Collections of Freiberg University of Mining and Technology, Faculty of Geosciences, Geotechnics and Mining, Freiberg, Germany (the inventory number 80251)

Sphaerobertrandite, Be3SiO4(OH)2: new data, crystal structure and genesis

The insufficiently studied beryllium silicate sphaerobertrandite has been known since 1957. The present work presents new findings, verifies that the mineral is a valid species with a unique structure. The original name sphaerobertrandite has been kept in spite of its unjustifiable connection to bertrandite. The mineral is named after the typical spherulitic morphology of its aggregates, and its similarity to bertrandite in the main chemical constituents. Sphaerobertrandite was recently found in alkaline pegmatites at Sengischorr Mountain, Lovozero massif, Kola peninsula, Russia, inside epididymite segregations, coexisting with eudidymite, aegirine, mangan-neptunite, etc., and in Tuften quarry, Tvedalen, South Norway, coexisting with hambergite, analcime, chiavennite, etc. The mineral forms spherulites up to 2 mm, as well as fibrous crusts. Sphaerobertrandite from Sengischorr Mountain occurs as thin tabular, prismatic crystals up to 0.5 × 0.2 × 0.05 mm inaggregates overgrowing epididymite. The main crystal form is {001}, small faces {012}, {102} and {10-2} are present. Transparent to translucent; colourless, white, yellow, brownish, greyish, beige. Streak white. Lustre vitreous. Mohs' hardness 5. Brittle. Cleavage perfect on (001). D (meas.) is 2.46 - 2.54, D (calc.) is 2.52 g/cm3. Biaxial, negative, alpha = 1.597(3), beta = 1.607(4), gamma = 1.616(3), 2V (meas) is 70(±20)° Orientationn: Z = c. IR spectrum is unique; frequencies of absorption bands are (cm-1; sh - shoulder, w - weak; the most intensive bands are underlined): 3605, 3540, 3505, 3370sh, 3250sh, 3060w, 1620sh, 1400sh, 1150, 1115, 1090sh, 995sh, 933, 900, 835, 768, 721, 680sh, 639, 612, 573w, 555w, 491, 424, 410sh. Chemical composition of the sample from Sengischorr Mountain is: BeO 45.88, SiO2 38.46, H2O+ 12.54, total 96.88 wt. %, corresponding to Be2.97Si1.03O4.06 (OH)1.94 • 0.155H2O. Monoclinic, P2llc, with a = 5.081(3), b = 4.639(1), c = 17.664(9) Å, beta = 106.09(5)°, V = 400.0 Å 3, with a strongly pseudo-orthorhombic cell, which is the likely reason for the samples being invariably twinned by pseudo-merohedry. The strongest lines in the X-ray powder pattern are (d in Å -I[hkl]): 4.885-90[100]; 4.236-62[004]; 3.161-100[111, 11-3]; 2.836-70[104]; 2.538-55[20-2]; 2.318-90[020]; 2.174-55[10-8]. The crystal structure has been refined to R = 0.090. Better refinement could not be achieved because of twinning by pseudo-merohedry with low obliquity, which produces partial overlapping of reflections. The structure consists of mixed framework formed by Be- and Si-tetrahedra which comprises 6- and 4- membered rings. All vertices of SiO4 tetrahedra are shared with 2 BeO4 tetrahedra. The OH anions are shared only between two Be tetrahedra. The structure contains Be(3)O3 chains screwed around 2 axis and reinforced by SiO4 tetrahedra. These (Be,Si,O) chains are linked by the dimers Be(2)2O6 and by Be(1)O4 tetrahedra