Burtite, calcium hexahydroxostannate, a new mineral from El Hamman, central Morocco

Burtite, CaSn(OH) (sub 6) , occurs as partially altered octahedra in late hydrothermal garnetite, situated in a tin skarn. The mineral is colourless with a good cubic cleavage, vitreous lustre, H. approx 3. Microprobe analysis gave SnO (sub 2) 56.3, CaO 20.6, MgO 0.3, = 77.2. Excess water may be structurally trapped. It is weakly birefringent in thick grains, uniaxial positive, n 1.633 + or - 0.001; sp. gr. 3.28. X-ray powder data indexed on the cubic pseudocell (a 8.128 A) are included, as are results of Gladstone-Dale calculations. The optical data indicate true rhombohedral symmetry, probable space group R3.