Abstract

Simmonsite, Na 2 LiAlF 6 , a new mineral of pegmatitic-hydrothermal origin, occurs in a late-stage breccia pipe structure that cuts the Zapot amazonite-topaz-zinnwaldite pegmatite located in the Gillis Range, Mineral Co., Nevada, U.S.A. The mineral is intimately intergrown with cryolite, cryolithionite and trace elpasolite. A secondary assemblage of other alumino-fluoride minerals and a second generation of cryolithionite has formed from the primary assemblage. The mineral is monoclinic, P2 1 or P2 1 /m, a = 7.5006(6)Aa, b = 7.474(1)Aa, c = 7.503(1)Aa, beta = 90.847(9) degrees , V = 420.6(1)Aa 3 , Z = 4. The four strongest diffraction maxima [d (Aa), hkl, I/I 100 ] are (4.33, 111 and 111, 100); (1.877, 400 and 004, 90); (2.25, 131, 113, 131 and 311, 70); and (2.65, 220, 202, 022, 60). Simmonsite is pale buff cream with white streak, somewhat greasy, translucent to transparent, Mohs hardness of 2.5-3, no distinct cleavage, subconchoidal fracture, no parting, not extremely brittle, D m is 3.05(2) g/cm 3 , and D c is 3.06(1) g/cm 3 . The mineral is biaxial, very nearly isotropic, N is 1.359(1) for lambda = 589 nm, and birefringence is 0.0009. Electron microprobe analyses gave (wt%) Na = 23.4, Al = 13.9, Al = 13.9, F = 58.6, Li = 3.56 (calculated), with a total of 99.46. The empirical formula (based on 6 F atoms) is Na (sub 1.98) Li (sub 1.00) Al (sub 1.00) F 6 . The crystal structure was not solved, presumably because of unit-cell scale twinning, but similarities to the perovskite-type structure exist. The mineral is named for William B. Simmons, Professor of Mineralogy and Petrology, University of New Orleans, New Orleans.

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