Brandaoite, [BeAl2(PO4)(2)(OH)(2)(H2O)(4)](H2O), a new Be-Al phosphate mineral from the Joao Firmino mine, Pomarolli farm region, Divino das Laranjeiras County, Minas Gerais State, Brazil: description and crystal structure

摘要

Brandaoite, [BeAl2(PO4)2(OH)2(H2O)4](H2O), is a new Be-Al phosphate mineral from the Joao 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 <10 mu m wide and <100 mu m 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. Brandaoite is biaxial (+), alpha = 1.544, beta = 1.552 and gamma = 1568, all +/- 0.002; 2Vobs = 69.7(10)degrees and 2Vcalc = 71.2 degrees. No pleochroism was observed. Brandaoite is triclinic, space group P (1) over bar, a = 6.100(4), b = 8.616(4), c = 10.261(5) angstrom, alpha = 93.191(11), beta = 95.120(11), gamma = 96.863(11)degrees, V=532.1(8) angstrom³ and Z= 2. Chemical analysis of a 4 mu m wide needle-shaped crystal by electron microprobe and secondaryion mass spectrometry gave P2O5 = 28.42, Al2O3 = 20.15, BeO = 4.85, H2O = 21.47 and sum = 74.89 wt.%. The empirical formula, normalised 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. Beryllium and P tetrahedra share corners to form a four-membered ring. Aluminium octahedra share a common vertex to form an [Al2phi11] diner, 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].