New Mexico Mineral Symposium — Abstracts

Foitite, the newest member of the tourmaline group, is the alkali-deficient and aluminous analog of schorl. It was originally described from two isolated crystals labeled merely "southern California" and has since been recognized as zones in hairlike fibers in cavities in pegmatite at the classic Elba locality of Filone della Speranza. A new foitite occurrence (third world occurrence) was identified at the Tungsten or Wichita mine on Copper Mountain, in the Picuris Range, Taos County, New Mexico, which is about 2¹/₂ mi northeast of the famous Harding mine. This is a wolframite deposit mentioned briefly by Hess in Tungsten mineials and deposits (U.S.G.S. Bulletin 652). Here foitite is found as brown, cross-fiber veins 2 cm wide in quartz. It was analyzed as part of a larger study by Dyar et al. (submitted to the American Mineralogist) by a combination of modern techniques to provide a thorough characterization of its chemistry. Methods included electron microprobe analysis for the major elements, proton-induced gamma-ray emission analysis (PIGE) for lithium, boron, and fluorine, Mossbauer spectroscopy for the ferrous/ferric ratio, and uranium extraction for hydrogen. The results yield a formula with no ferric iron, only 0.35 sodium atoms in the X site, ferrous iron and aluminum dominating the Y site, and a deficiency of 1.37 protons. This is an unusual tourmaline, clearly a foitite, and the data suggests that both the alkali-defect and dehydroxylation substitution mechanisms identified by Foit and Rosenberg (1977) are operative in this sample.