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The breccias in the Fifes Peak are mostly shades of gray, purple, and red, with angular to subrounded rock fragments cemented by ash. The fragments are a few inches in dimension, rarely a foot long. They are mostly andesite porphyry, pumice, and glass. With fewer rock fragments, the breccias is a buff and tan. Pieces of petrified wood may be ...
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
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.
Andesine is a silicate mineral, a member of the plagioclase feldspar solid solution series. Its chemical formula is ( Ca , Na )( Al , Si ) 4 O 8 , where Ca/(Ca + Na) (% anorthite ) is between 30 and 50%.
Rock richer in alkali feldspar is classified as syenite, while rock richer in plagioclase is termed a monzodiorite. The volcanic equivalent of monzonite is latite. [2] [4] The plagioclase in monzonite is sodium-rich, ranging from oligoclase to andesine, and is moderately well shaped (subhedral to euhedral). [5] The alkali feldspar is typically ...
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]
Quartz (SiO 2) is present as more than 20% of the total quartz-alkali feldspar-plagioclase-feldspathoid content of the rock. [ 1 ] [ 2 ] Amphiboles and biotite are common in lesser quantities, while accessory minerals include apatite , magnetite and zircon .
Anorthosite (/ ə ˈ n ɔːr θ ə s aɪ t /) is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%).