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28_e9750264d5476c85f9be09d17bc6fd51
2005-05-26 12:03:39

Biotite is a common phyllosilicate mineral that contains potassium, magnesium, iron and aluminium. It is sometimes called "iron mica" and is found in granitic rocks, gneisses, and schists. Like other mica minerals, biotite has a highly perfect basal cleavage, its flexible sheets easily flaking off. It has a hardness of 2.5 - 3, a specific gravity of 2.7 - 3.1, is colored greenish to brown or black, and can be transparent to opaque. Biotite is occasionally found in large sheets, especially in...

28_7ea4778c81fd8cde7aa462546b2ebb04
2005-05-26 10:50:45

Gneiss is a common and widely distributed type of rock formed by high grade regional metamorphic processes from preexisting formations that were originally either igneous or sedimentary rocks. Gneissic rocks are coarsely laminated and largely recrystallized but do not carry large quantities of micas, chlorite or other platy minerals. Gneisses that are metamorphosed igneous rocks or their equivalent are termed granite gneisses, diorite gneisses, etc. However, depending on their composition,...

28_8002f27f8f96dedfc5d591fb18ec14b5
2005-05-26 09:41:33

The mineral kyanite is an aluminium silicate of the sillimanite group (along with andalusite and sillimanite), also called alumino-silicate. It has the formula Al2O3·SiO2 (Al2SiO5). Its hardness varies widely depending on its crystallographic direction, from 5 to 7.5, and it has a specific gravity of 3.56-3.67. It is translucent, usually blue, but possibly white, gray, green, or black. It is usually found in long-bladed crystals or columnar to fibrous structures. Kyanite is used in...

28_b2a76c7ad7733834d7931b9a4d167359
2005-05-25 18:35:54

The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. By definition, schist contains more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. The individual mineral grains in schist, drawn out into flaky scales...

28_af89e4efb1af170ba5cc9f34e6ddc5fb
2005-05-25 17:09:09

Slate is a fine-grained, homogeneous, sedimentary rock composed of clay or volcanic ash which has been metamorphosed (foliated) in layers (bedded deposits). Slate can be made into roofing shingles ('roofing slates' in the United Kingdom), because it has two lines of breakability: cleavage and grain. This makes it possible to split slate into thin sheets. Slate is mainly composed of quartz and muscovite (a mica), often along with biotite, chlorite, and hematite, or, less frequently,...

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Word of the Day
kenspeckle
  • Having so marked an appearance as easily to be recognized.
This word may come from the Swedish 'kanspak,' quick at recognizing persons or things, or else from confusion with 'conspicuous.'