Tobelite, a new ammonium dioctahedral mica.

Abstract

Mineralogical properties of tobelite, a new ammonium-dominant dioctahedral mica, found in the Ohgidani pottery stone deposit at Tobe, Ehime Prefecture, Japan, are described.This mineral, accompanied by quartz, occurs as a hydrothermal alteration product of a biotite andesite dyke. Wet chemical analysis gives a structural formula: ((NH4)0.53K0.19Na0.01 0.27)1.00 (Al1.97Ti0.00Fe3+0.03Mg0.05)2.05(Si3.17Al0.83)4.00O10(OH)2. The low interlayer charge is explained with its poorly crystallized nature. The X-ray powder diffraction pattern is very close to that of synthetic NH4Al2Si3AlO10(OH)2 by Eugster and Munoz (1966), and is satisfactorily indexed on 1M polytype cell having a=5.219A, b=8.986A, c=10.447A and β=101.31°. Space group may be C2⁄m. Tobelite is characterized by its larger unit layer thickness (10.25A) than that of potassium dioctahedral mica. This material yields an endothermic reaction and absorption bands specific to ammonium in DTA curve and infrared spectrum, respectively. Optically it is biaxial, negative, 2Vcalc.=28°. Refractive indices are α=1.555, β=1.575, γ=1.581, all±0.002.Mineralogical description is made also on tobelite from the Horo pyrophyllite deposit, Hiroshima Prefecture, Japan, which is well crystallized as compared with the Tobe material and is tentatively determined to be of 2M2 polytype.