Surinamite, a new Mg-Al silicate from the Bakhuis Mountains, western Surinam; I, Description, occurrence, and conditions of formation Available to Purchase

Surinamite, (Al_1.38Mg_1.12Fe_0.46Mn_0.04)^VI (Si_1.51Al_0.49)O_7.36(OH)_0.64, is monoclinic, P2₁/a; a = 9.64, b = 11.36, c = 4.95 Å, β = 109,0°, V = 512.5 ų, Z = 4, ρ_calc = 3.58; strongest powder lines at 2.435, 1.99, 1.420, and 2.91 Å; α = 1.738, β = 1.743, γ = 1.746, 2V_α = 67-68° b = Y; very strong dispersion such that in (010) the extinction angle between Z and the trace of a cleavage perpendicular to (010) is 31° for violet and 44° for yellow light; pleochroism, Y = violet, for vibrations in the optic plane (010), bright blue-green if parallel to the cleavage but very light-colored if perpendicular thereto; formed as minute (<0.2 mm) crystals, tabular on {010}, in a mylonitic spinel-, kyanite-, and sillimanite-bearing mesoperthite gneiss. The surinamite occurs in aggregates with biotite, kyanite, and sillimanite, which are probably pseu-domorphs after cordierite, and formed under higher pressure granulite-facies conditions. Surinamite resembles sapphirine optically, in X-ray powder pattern, and in structure. A proposed crystal structure involving dense-packed oxide sheets as in sapphirine satisfactorily accounts for the spinel-like substructure reflections.