A Geoscience Digital Image Library

TitleThin section of dacite
DescriptionThis is a plain polarized thin section of dacite. A markedly porphyritic rock with a uniformly fine-grained holocrystalline matrix. The phenocrysts are chiefly strongly zoned, intermediate plagioclase and a few subrounded, partly resorbed grains of sanidine. The microphenocrysts are brown biotite and green hornblende, the latter showing minor marginal resorbtion rims. The matrix consists chiefly of a very fine-grained quartz-feldspar intergrowth and scattered patches of wispy biotite.
ChronostratigraphyLate Cretaceous
LocationUSA ▹ Montana. Near Helena.
PhotographerDarla Sondrol. 2002-04-09.
CollectionWard’s Collection of Classic North American Rocks #26.
Key wordsdacite, Montana, thin section
Tech details1023 KB. Thin section. Nikon FDX-35.
GeoDIL number2632