Ganterite, the barium mica Ba_0.5K_0.5Al₂(Al_1.5 Si_2.5)O₁₀(OH)₂, from Oreana, Nevada

Chi Ma and George R. Rossman

Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA 91125-2500, USA

ABSTRACT

The barium dioctahedral layer silicate, ganterite, was identified from the Lincoln Hill dumortierite deposit near Oreana, Humbolt County, Nevada, based on electron microprobe, electron back scatter diffraction (EBSD) and Raman spectrum micro-analyses. This phase occurs with dumortierite, barite and muscovite in a vein specimen formed by hydrothermal alteration. Backscatter electron images of the muscovite from this locality show extensive zonation of the BaO content with regions of high Ba concentrations of up to 15 µm in dimension. Electron microprobe analyses of these regions reveal a composition (Ba_0.53K_0.37Na_0.05)_sum=0.95 (Al_2.00Ti_0.01)_sum=2.01 [Al_1.51Si_2.49O₁₀] (OH)₂ or, ideally, (Ba_0.5K_0.5) Al₂ (Al_1.5Si_2.5) O₁₀ (OH)₂. This composition corresponds to the recently described mica, ganterite. Complete solid solutions between muscovite and ganterite were observed that range from 0.60% up to 18.12 wt% BaO. The electron backscatter diffraction and Raman spectra of this phase are essentially indistinguishable from those of muscovite confirming that ganterite has a mica structure.

The original hand specimen, CIT-1547, from the Lincoln Hill dumortierite deposit near Oreana, Nevada.

Plot of compositions on a Ba K, Na diagram