Abstract

Two new opaque minerals from metasomatized peridotites and kimberlites in the Republic of South Africa. Ba and K members of the AM 21 O 38 series, characterized by predominant A-specific large cations and M-formula small cations. Electron microbeam analyses give the following formulae: lindsleyite: A = Ba (sub 0.62) Sr (sub 0.41) Ca (sub 0.09) Pb (sub 0.01) K (sub 0.07) Na (sub 0.06) REE (sub 0.03) , M = Ti (sub 11.63) Zr (sub 0.87) Al (sub 0.11) Cr (sub 3.89) Fe (sub 2.87) Mg (sub 1.49) Nb (sub 0.08) ; mathiasite: A = K (sub 0.66) Ca (sub 0.22) Sr (sub 0.15) Ba (sub 0.10) Na (sub 0.05) REE (sub 0.05) , M = Ti (sub 13.18) Zr (sub 0.62) Cr (sub 2.87) Fe (sub 2.28) Mg (sub 1.63) Ca (sub 0.22) Nb (sub 0.09) . Space group is R3. Hexagonal unit cell parameters for lindsleyite are a = 1.037 nm, c = 2.052 nm, V = 1.911 nm 3 , and for mathiasite are a = 1.035 nm, c = 2.058 nm, V = 1.909 nm 3 . Calculated densities for the Ba and K members are 4.63 and 4.60 gm/cm 3 . Both minerals are black and have a metallic luster and conchoidal fracture. The minerals are pale tan in reflected light, exhibit weak reflection pleochroism and weak-moderate reflection anisotropy. Spectral reflectances for lindsleyite and mathiasite are very similar and are comparable to ilmenite and rutile.--Modified journal abstract.

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