Surinamite, (Al1.38Mg1.12Fe0.46Mn0.04)VI (Si1.51Al0.49)O7.36(OH)0.64, is monoclinic, P21/a; a = 9.64, b = 11.36, c = 4.95 Å, β = 109,0°, V = 512.5 Å3, Z = 4, ρcalc = 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, 2Vα = 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.

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