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Diorite is an intrusive igneous rock composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and sometimes pyroxene.The chemical composition of diorite is intermediate, between that of mafic gabbro and felsic granite.
Hornblende diorite from the Henry Mountains, Utah, US. Hornblende is a common constituent of many igneous and metamorphic rocks such as granite, syenite, diorite, gabbro, basalt, andesite, gneiss, and schist. It crystallizes in preference to pyroxene minerals from cooler magma that is richer in silica and water. [13]
The formation varies from hornblende diorite and quartz diorite in the lowest part to albite-rich granite and syenite in the upper part. Within the formation large phenocrysts of hornblende, orthoclase and garnet contribute to distinctive appearance. [1] The formation was first named by William J. Miller as "Mount Lowe Granodiorite" in 1934. [2]
Amphibolite from Cape Cod, Massachusetts Garnet bearing amphibolite from Val di Fleres, Italy. Amphibolite (/ æ m ˈ f ɪ b ə l aɪ t /) is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz.
Hornblende crystallises in the monoclinic system and occurs as black crystals or grains in many different types of igneous and metamorphic rocks, including hornblende-granite, syenite, diorite, andesite, hornblende-schist, and amphibole. hornfels A hard, compact, fine-textured contact-altered argillaceous rock that breaks into splintery ...
It is described as a "Moderately to strongly deformed intrusive complex, chiefly composed of quartz diorite gneiss. Rock types include gneissic biotite-quartz diorite, hornblende-biotite-quartz diorite, and biotite granodiorite, with minor amounts of quartz monzonite and hornblende-quartz diorite.
Biotite and amphiboles often in the form of hornblende are more abundant in granodiorite than in granite, giving it a more distinct two-toned or overall darker appearance. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals.
Embedded in the diorite are a couple of large gabbro inclusions, as well as a picrite on the east of Braye Bay. Some of the diorite has orbicular structure, concentric spheres of plagioclase and hornblende rich zones form balls up to 20 cm in diameter. A pale coloured granite intruded on the north: the Bibette Head Granite.