Andremeyerite, a new barium iron silicate, from Nyiragongo, Zaire

Abstract

This paper presents a new silicate, andremeyerite, from the Nyiragongo volcano in Zaire. The mineral occurs in the vesicles of a melilite-leucite-nephelinite block carried up by later nephelinite lava. The constituents of the vesicles are briefly described. Andremeyerite is monoclinic, space group P21\c. Unit cell: a0 = 7.464 Å, b0 = 13.794 Å, c0 = 7.093 Å, ß = 118° 15'. Indexed powder pattern given. Crystals prismatic with (100), (120), (010), (011) simulating tetragonal shape. Perfect cleavage on (100) and (010). Multiple twinning on (100). Density 4.15. Vickers indentation microhardness 440 kg/mm2. Strong dispersion. Optical orientation: y || b, c /\ a varying from 2° (for 670 mμ) to 61° (for 470 mμ). Position of the optic axes given in stereographic projection. Optic sign +. Overall color pale emerald green. Pleochroism in daylight: α = slightly bluish green, ß~γ = colorless with a faint brownish tint. Refractive indices in sodium light: α~ß = 1.740, γ = 1.760. Idealized chemical formula BaFe2Si2O7 with Z = 4. The iron is probably mainly in the ferrous state. The mineral starts decomposing at ca. 800° C.