Untitled research work

Chemical data on amphibole phenocrysts in andesitic to rhyolitic rocks have been compiled mainly from the literature. It is inferred that most amphibole phenocrysts have been oxidized more or less by the loss of hydrogen in connection with volcanic activities from the fact that the volcanic amphiboles are generally higher in Fe3+/ΣFe ratio and lower in H2O+ content than their plutonic equivalents. Bimodal distribution of Fe3+/ΣFe ratio in the phenocrysts suggests that change of “usual amphibole” into “oxyhornblende” is very rapid. Kaersutite, usually considered to be confined to alkaline rocks, seems to occur in some calc-alkaline volcanics. Assuming that the original Fe3+/ΣFe ratio was 0.15, the numbers and ratios of cations in primary amphiboles are frequently in the following ranges on the basis of O=23 (average in parenthesis): Si=6.2-7.0 (6.5), AlIV=1.0-1.8 (1.5), AlVI=0.0-0.5 (0.3), Ti=0.1-0.3 (0.2), ΣFe=1.4-2.2 (1.7), Mn=0.01-0.06 (0.04), Mg=2.6-3.4 (2.9), Ca=1.6-1.9 (1.8), Na=0.3-0.8 (0.5), K=0.0-0.2 (0.1), Na+ K=0.4-0.9 (0.6), mg=0.5-0.8 (0.6) and K/Na=0.0-0.4 (0.2).