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A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than 1 cm (0.4 in) and sometimes greater than 1 meter (3 ft). Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic composition to granite.
This is typical of pegmatites. Pegmatites are most commonly formed as coarse-grained igneous rocks of granitic composition, containing large clasts of gemstones such as amazonite, garnet, and topaz. Phaneritic (phaner = visible) textures are typical of intrusive igneous rocks, these rocks crystallized slowly below Earth's surface.
Granite often occurs as relatively small, less than 100 km 2 stock masses and in batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions. In some locations, very coarse-grained pegmatite masses occur with granite. [19]
Granite (light) with sheeted veins of greisen (dark) at Cligga Head, Cornwall. Greisen is a highly altered granitic rock or pegmatite, usually composed predominantly of quartz and micas (mostly muscovite). Greisen is formed by self-generated alteration of a granite and is a class of moderate- to high-temperature magmatic-hydrothermal alteration ...
Macroscopic aspects of nepheline syenite are similar to those of granite. The presence of nepheline and absence of quartz are the fundamental difference. Biotite is generally of low content and the main mafic minerals are clinopyroxene (±) and amphibole (±). The macroscopic colour is grey, being little darker than granite.
The Sebago Granite makes up the Sebago Pluton, which is itself a part of the Sebago Migmatite Domaine. [9] The larger Sebago Batholith is composed of both the Sebago Granite and the Sebago Pegmatite System. [2] The pegmatites from the Sebago Pegmatite Group surround the granite within the batholith. These two units differ in grain size, as well ...
The original name for this phenomenon was defined by Sederholm (1923) [24] as a rock with "fragments of older rock cemented by granite", and was regarded by him to be a type of migmatite. There is a close connection between migmatites and the occurrence of ‘explosion breccias’ in schists and phyllites adjacent to diorite and granite intrusions.
Migmatite is a gneiss consisting of two or more distinct rock types, one of which has the appearance of an ordinary gneiss (the mesosome), and another of which has the appearance of an intrusive rock such pegmatite, aplite, or granite (the leucosome). The rock may also contain a melanosome of mafic rock complementary to the leucosome. [11]