gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1931.022.134.02 Notes on Brown Hornblende and Biotite from Shab&:o-Zan, of the Daiton Volcanoes, Taiwan, Japan Ichimura Takeshi Professor of Geology, Taihoku Imperial University, Taiwan, Japan 9 1931 22 134 561 568 Copyright © 1931, The Mineralogical Society 1931 Summary

So far as known at present the basaltic hornblende is mostly contained in the grey variety of andesite and its noritic inclusions.

Brown hornblende, on the other hand, predominantly occurs in brown andesite.

The occurrence of the brown biotite is not known in the andesite: it is restricted only to the noritic inclusions.

There are various stages of alteration among these hornblendes, gradually changing the pleochroism.

Both hornblende and biotite show a similar pleochroic scheme in their later stages: hypersthene, augite, and magnetite finally change into haematite during this process.

The hornblende shows an increase in the index of refraction and birefringence, while the extinction-angle decreases gradually. Chemically, the brown hornblende shows a conspicuous decrease in FeO.

Brown hornblende and biotite were derived from basaltic varieties, as shown by various transitions among them. It is evident that the brown hornblende here has not come from a green variety.

The formation of brown hornblende and biotite may be due to the oxidation of basaltic varieties, probably under a certain condition of reheating where FeO passes into Fe2O3.