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Darker andesite can be challenging to distinguish from basalt, but a common rule of thumb, used away from the laboratory, is that andesite has a color index less than 35. [9] The plagioclase in andesite varies widely in sodium content, from anorthite to oligoclase, but is typically andesine, in which anorthite makes up about 40 mol% of
Porphyritic texture is particularly common in andesite, with the most prominent phenocrysts typically composed of plagioclase feldspar. [9] [10] Plagioclase has almost the same density as basaltic magma, so plagioclase phenocrysts are likely to remain suspended in the magma rather than settling out. [11]
Plagioclase displaying cleavage. (unknown scale) In volcanic rocks, fine-grained plagioclase can display a "microlitic" texture of many small crystals. Plagioclase (/ ˈ p l æ dʒ (i) ə ˌ k l eɪ s, ˈ p l eɪ dʒ-,-ˌ k l eɪ z / PLAJ-(ee)-ə-klayss, PLAYJ-, -klayz) [4] is a series of tectosilicate (framework silicate) minerals within the feldspar group.
Basaltic andesite from Parícutin volcano in Mexico. Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central America [1] and the Andes of South America. [2]
Fifes Peaks Formation consists of flows that are vesicular, basaltic andesite. Weathering the rock creates shades of dark brown. The newly fractured blocks are medium- to dark-gray colors. Some flows are fine grained, most are porphyritic and contain glassy phenocrysts of plagioclase that weather to chalky-appearing
For example, porphyritic andesite will have large phenocrysts of plagioclase in a fine-grained matrix. Also in South Africa , diamonds are often mined from a matrix of weathered clay -like rock ( kimberlite ) called "yellow ground".
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It has a fine-grained to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz. Dacite is relatively common, occurring in many tectonic settings.