GEOL 3041 Chapter : 3041 SI 2
Document Summary
Megascopic description: generally grey (on a fresh surface), medium-grained phaneritic dacite with large phenocrysts of tridymite. Dacite is chiefly composed of plagioclase, orthoclase and pyroxene. Plagioclase (50%) [50-1250 m] euhedral grains shown in low relief with normal zoning at margins. Orthoclase (20%) [100-500 m] anhedral crystals of orthoclase are the last to crystalize in the paragenic sequence. Tridymite (15%) [100-5625 m] large euhedral crystals are shown in low relief. Locally some tridymite crystals are replaced by orthoclase. Pyroxene (15%) [50-600 m] subhedral grains, yellow to gold in color, are sited in high relief. Pyroxene is being replaced by orthoclase in some cases. Magnetite (tr) [50-100 m] anhedral opaque black grains exhibiting low relief. Textures: euhedral tridymite and plagioclase phenocrysts largely denote an idiomorphic-granular texture. Alteration: replacement of tridymite and pyroxene by orthoclase is locally present. The rock is best termed a tridymite dacite as designated by the qapf ratio of 15:20:50 for the primary volcanic mineral grouping.
