Abstract
Phenocrysts of Ca-Fe-rich pyroxenes such as ferroaugite and ferrohedenbergite were found in perlite lavas, which occur in the Middle Miocene formations in the Tsugawa and Tadami areas, although many of acid volcanic rocks in the areas consist of phenocrysts of biotite or hornblende. These perlites consist of phenocrysts of plagioclase (andesine-oligoclase), ferroaugite or ferrohedenbergite, altered olivine and titanomagnetite in a pale brown glassy groundmass.
Ferroaugite and ferrohedenbergite occur in the British and Icelandic Tertiary acid glass and the late stages of the Skaergaard and Bushveld intrusions. Such pyroxenes also occur in some alkaline rocks. On the Ca-Mg-Fe quadilateral, chemical compositions of Ca-Fe-rich pyroxenes from the perlites are plotted near the trend curve of pyroxenes of tholeiitic rocks and are lower TiO2, A12O3 and K2O contents than those from alkaline rocks.
