IMA 2016-071a = brandãoite

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Marco E. Ciriotti
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IMA 2016-071a = brandãoite

Messaggio da Marco E. Ciriotti » lun 02 lug, 2018 11:13

Prossima pubblicazione.

▪ Menezes Filho, L.A.D., Chaves, M.L.S.C., Cooper, M.A., Ball, N.A., Abdu, Y.A., Sharpe, R., Day, M.C., Hawthorne, F.C. (2018): Brandãoite, [BeAl2(PO4)2(OH)2(H2O)4](H2O), a new Be-Al-phosphate mineral from the João Firmino mine, Pomarolli farm region, Divino das Laranjeiras county, Minas Gerais State, Brazil: description and crystal structure. Mineralogical Magazine, 82, (in press).

Brandãoite, [BeAl2(PO4)2(OH)2(H2O)4](H2O), is a new Be-Al-phosphate mineral from the João Firmino mine, Pomarolli farm region, Divino das Laranjeiras County, Minas Gerais State, Brazil, where it occurs in an albite pocket with other secondary phosphates, including beryllonite, atencioite and zanazziite, in a granitic pegmatite. It occurs as colourless acicular crystals less than 10 μm wide and less than 100 μm long that form compact radiating spherical aggregates up to 1.0-1.5 mm across. It is colourless and transparent in single crystals and white in aggregates, has a white streak and a vitreous lustre, is brittle and has conchoidal fracture. Mohs hardness is 6, and the calculated density is 2.353 g/cm3. Brandãoite is biaxial positive, α = 1.544, β = 1.552, γ = 1.568, all 0.002; 2Vobs = 69.7(10)°, 2Vcalc = 71.2°. No pleochroism was observed. Brandãoite is triclinic, space group P-1, a = 6.100(4), b = 8.616(4), c = 10.261(5) Å, α = 93.191(11), β = 95.120(11), γ = 96.863(11)°, V = 532.1(8) Å3, Z = 2. Chemical analysis of a needle 4 μm wide by electron microprobe and secondary-ion mass spectrometry gave P2O5 = 28.42, Al2O3 20.15, BeO 4.85, H2O 21.47, sum 74.89 wt%. The empirical formula, normalized on the basis of 15 anions pfu with (OH) = 2 and (H2O) = 5 apfu (from the crystal structure) is Be0.98Al1.99P2.02H12O15. The crystal structure was solved by direct methods and refined to an R1 index of 7.0%. There are two P sites occupied by P5+, two Al sites occupied by octahedrally coordinated Al3+, and one Be site occupied by tetrahedrally coordinated Be2+. There are fifteen anions, two of which are (OH) groups and five of which are (H2O) groups. The simplified ideal formula is thus [BeAl2(PO4)2(OH)2(H2O)4](H2O) with Z = 2. Be and P tetrahedra share corners to form a four-membered ring. Al octahedra share a common vertex to form an [Al2φ11] dimer, and these dimers are cross-linked by P tetrahedra to form a complex slab of polyhedra parallel to (001). These slabs are cross-linked by BeO2(OH)(H2O) tetrahedra, with interstitial (H2O) groups in channels that extend along [100].
Marco E. Ciriotti

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