|
Schist is a medium-grade metamorphic rock〔(【引用サイトリンク】title=Schist definition )〕 with medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. These lamellar (flat, planar) minerals include 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. Schist is often garnetiferous. Schist forms at a higher temperature and has larger grains than phyllite.〔 Geological foliation (metamorphic arrangement in layers) with medium to large grained flakes in a preferred sheetlike orientation is called ''schistosity''.〔Essentials of Geology, 3rd Ed, Stephen Marshak〕 The names of various schists are derived from their mineral constituents. For example, schists rich in mica are called mica schists and include biotite or muscovite.〔 Most schists are mica schists, but graphite and chlorite schists are also common. Schists are also named for their prominent or perhaps unusual mineral constituents, as in the case of garnet schist, tourmaline schist, and glaucophane schist. The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen with the naked eye. Schist is characteristically ''foliated'', meaning that the individual mineral grains split off easily into flakes or slabs. The word schist is derived ultimately from the Greek word ''σχίζειν schízein'' meaning "to split", which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie. Most schists are derived from clays and muds that have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists are derived from fine-grained igneous rocks such as basalts and tuffs. Schists are frequently used as dimension stone, which is stone that has been selected and fabricated to specific shapes or sizes. ==Historical mining terminology== Before the mid-18th century, the terms slate, shale and schist were not sharply differentiated by those involved with mining.〔R. W. Raymond, Slate, (A Glossary of Mining and Metallurigical Terms ), American Institute of Mining Engineers, 1881; page 78.〕 In the context of underground coal mining, shale was frequently referred to as slate well into the 20th century.〔Albert H. Fay, Slate, (A Glossary of the Mining and Mineral Industry ), United States Bureau of Mines, 1920; page 622.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「schist」の詳細全文を読む スポンサード リンク
|