Notes on Brown Hornblende and Biotite from Shabo-Zan, of the Daiton Volcanoes, Taiwan, Japan

Takeshi Ichimura
Professor of Geology, Taihoku Imperial University, Taiwan, Japan

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.

Mineralogical Magazine; September 1931 v. 22; no. 134; p. 561-568; DOI: 10.1180/minmag.1931.022.134.02
© 1931, The Mineralogical Society
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