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

This paper presents a new silicate, andremeyerite, from the Nyiragongo volcano in Zaire. The mineral occurs in the vesicles of a melilite-leucitenephdinite block carried up by later nephelinite lava. The constituents of the vesicles are briefly described. Andremeyerite is monoclinic, space group P2x\c. Unit cell: aa = 7.464 A, bo = 13.794 A, co = 7.093 A, ß = 118° 15'. Indexed powder p°attern 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 a varying from 2° (for 670 m/i) to 61° (for 470 m/i). Position of the optic axes given in stereographic projection. Optic sign + . Overall color pale emerald green. Pleochroism in daylight: a = slightly bluish green, ß •—y = colorless with a faint brownish tint. Refractive indices in sodium light: a~ß = 1.740, y = 1.760. Idealized chemical formula BaFe2Si207 with Z = 4. The iron is probably mainly in the ferrous state. The mineral starts decomposing at ca. 800° C.