Composition of Hydrothermal and Igneous Biotites, and Their Occurrence in Borehole No. 1, Dieng, Indonesia

Abstract

Textural relations and mineral chemistry have been used to determine the differences between igneous and hydrothermal biotites from different depths in borehole No. Dieng, Indonesia. The igneous and hydrothermal biotites occur as coarse and fine grainedmaterial Igneous are homogeneous while hydrothermal biotites change in composition with increasing temperature and depth. Hydrothermal biotites are depleted in Ti, Mg, and K,but enriched in Fe, Si, and Ca relative to igneous ones. The hydrothermal biotites also have sufficient (Si + Al) to fill the tetrahedral sites as indicated in the general structural Compositional variations of hydrothermal biotite reveal that they have been controlled by temperatures, rock permeability and duration of thermal activity.