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

Abstract

Foitite, anX-site-vacant tourmaline, occurs in both cross-fiber and slip-fiber textures with scheelite and wolframite in a quartzvein cutting metaquartzite of the Proterozoic Ortega Formation at the Tungsten (or Wichita) mine on Copper Mountain, PicurisRange, Taos County, New Mexico. The tourmaline is brown, and its indices of refraction e = 1.634(2) and o = 1.666(2). It isstrongly pleochroic, with E brown and O colorless. The density is 3.17(2) (meas.) and 3.20 glcm3 (calc.). Unircell parametersrefined from powder X-ray data are a 15.973,l), c 7.I37(I) A. and V 1576.8(2) Ar. The atomic arrangement of the CopperMountain foitite was refined to R = 0.018 using three-dimensional X-ray data. The sffuctural study confirms the low occupancyof the X site, and demonstrates that the atomic arrangement is essentially identical to that of the type material from southernCalifornia. Mtjssbauer specftoscopy shows that the iron is entirely ferrous, which indicates that the foitite crystallized at anoxygen fugacity below that of the quarlz - fayalite - iron buffer and, perhaps, as low as that of the iron wiistite buffer.Keywords: foitite, tourmaline, chemical composition, crystal structure, Copper Mountain, New Mexico.