Vermiculitization Process of Biotite from the Shinshiro Tonalite in the Eastern Part of Aichi Prefecture, Central Japan

Abstract

Vermiculitization process of biotite from a weathering profile of the Cretaceous Shinshiro tonalite was examined by means of X-ray diffraction, exchangeable inter-layer cation analysis and electron microprobe analysis. Biotite from the fresh rock shows a sharp 10Å basal reflection, but its Na-and Ca-modifications give distinct two basal reflections: 10 and 12.5Å for Na-modification and 10 and 15.3Å for Ca-modification. The exchangeable inter-layer cations, dominantly K and Mg, amount to 15.3meq/100g. The biotite from the fresh rock, in fact, contains microdomains of K·Mg-vermiculite. The basal refrections of the Na-and Ca-modifications broaden as the weathering increases. The exchangeable inter-layer cations are dominated by Ca in most weathered biotite, but the amount remains unchanged (15.0-19.0meq/100g) through the samples. The K·Mg-vermiculite formed at the initial stage of weathering changes to an interstratification of Ca-vermiculite and biotite with counterbalanced change of biotite to an interstratification of biotite and Ca-vermiculite.
Electron microprobe analyses including the FeLβ/FeLα intensity measurment show that Low-K portions possess lower iron and Fe²⁺/(Fe²⁺+Fe³⁺) ratio than do High-K portions even in a single biotite grain from the fresh rock. Weathered biotite from saprolites shows low Fe²⁺/(Fe²⁺+Fe³⁺) ratios. Oxidation of Fe²⁺ to Fe³⁺ in octahedral sheet, presumably causes the iron and K loss to maintain local electrostatic balance, and results in the transformation of biotite into dioctahedral K·Mg-vermiculite. The K-Ca exchange in inter-layer sites took place simultaneously with the transformation of the K·Mg-vermiculite to an interstratification of Ca-vermiculite and biotite. This study demonstrates that oxidation of Fe²⁺ to Fe³⁺ in octahedral sites acts as the trigger for vermiculitization of biotite by weathering.