Tobelite, a new ammonium dioctahedral mica.

Abstract

The 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 hydrothermal alteration product of a biotite andesite dyke. Wet chemical analysis gives a structural formula: (NH4)0.53K0.19Na0.01box 0.27)1.00(Al1.97Ti0.00 Fe3+0.03Mg0.05)2.05(Si3.17Al0.83)4.00O10(OH)2. The low interlayer charge is explained by its poorly crystallized nature. The XRD pattern is very close to that of synthetic NH4Al2Si3AlO10(OH)2 by Eugster & Munoz (1966), and is satisfactorily indexed on a 1M polytype cell having a 5.219, b 8.986, c 10.447 A, beta 101.31o; space group may be C2/m. Tobelite is characterized by its larger unit-layer thickness (10.25 A) than that of potassium dioctahedral mica. This material yields an endothermic reaction and absorption bands specific to ammonium in its DTA curve and IR spectrum, respectively. Optically it is biaxial, negative, 2Vcalc. 28o. Refractive indices are alpha 1.555, beta 1.575, gamma 1.581, (all + or -0.002). The name is for the locality. A description is also given for 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.-T.Y.