The occurrence and crystal structure of foitite from a tungsten-bearing vein at Copper Mountain, Taos County, New Mexico

Foitite, an X-site-vacant tourmaline, occurs in both cross-fiber and slip-f iber textures with scheelite and wolframite in a quartz vein cutting metaqu artzite of the Proterozoic Ortega Formation at the Tungsten (or Wichita) mi ne on Copper Mountain, Picuris Range, Taos County, New Mexico. The tourmali ne is brown, and its indices of refraction epsilon = 1.634(2) and omega = 1 .666(2). It is strongly pleochroic, with E brown and O colorless. The densi ty is 3.17(2) (meas.) and 3.20 g/cm(3) (calc.). Unit-cell parameters refine d from powder X-ray data are a 15.973(1), c 7.137(1) Angstrom, and V 1576.8 (2) Angstrom(3). The atomic arrangement of the Copper Mountain foitite was refined to R = 0.018 using three-dimensional X-ray data. The structural stu dy confirms the low occupancy of the X site, and demonstrates that the atom ic arrangement is essentially identical to that of the type material from s outhern California. Mossbauer spectroscopy shows that the iron is entirely ferrous, which indicates that the foitite crystallized at an oxygen fugacit y below that of the quartz - fayalite - iron buffer and, perhaps, as low as that of the iron - wustite buffer.