Augite from Hontanigoe of Aso-Caldera, Kumamoto

Abstract

The crystals of augite were found in hypersthene augite andesite lava at Hontanigce of Aso-caldera, Kumamoto Prefecture. This area is composed of seven lavas of two-pyroxene andesite and its agglomerates, which are observed at the inside wall of somma. At Hontanigoe, a dyke is observed which strikes N70°E. This dyke is hornblende andesite and is very rich in large hornblende phenocrysts that often attain 1 or 2cm. in length. The crystals of augite are greenish black in colour and idiomorphic with crystal faces a (100), b (010), m (110), and s (111), as shown Fig. I. The crystals of augite with shown a interpenetration twin have the following crystal faees; a (100), b (010), c (001), m (110), s (111), and p (101), as shown in Fig. 2. This augite has the following opitical properties: α=I.686-I.689, β=I.692-I.694, γ=I.710-I.713, 2V (+) =57°-59°. Colour is yellowish green and its pleochroism is weak in thin section, X=pale green, Y=pale yellow, Z=yellowish pale green. The crystals of augite show the hcurglass texture and develop the zonal structure. Inclusions of augite are irregular magnetite and. idiomcrphic plagioclase (An. 57-62%). The result of chemical analysis is shownin Table I. It is characteristic in its chemical constitution that this augite is rich in Al3O3 and poor in CaO. The writer presumes that this is related to the type of twinning and its opitical properties. The formula of the augite W. A. Deer & L. R. Wager follow: (XY)1•970 ((Si, Al)2O6). The weight % as wollastonite (CaSiO3), enstatite(MgSiO3), and ferrosilite (FeSiO3) are Wo: 39. 13%, En: 34.78%, Fs: 26.19%. In Table 2 is the constitutions as metasilicate. In addition to, the Table 3 is the bulk chemical composition of the hypersthene augite andesite and the chemical composition of the hypersthene hyenccryst.