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