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Talc (derived from (ペルシア語:تالک) ''tālk''; (アラビア語:طلق) ''ṭalq'') is a mineral composed of hydrated magnesium silicate with the chemical formula H2Mg3(SiO3)4 or Mg3Si4O10(OH)2. In loose form, it is the widely used substance known as baby powder (aka talcum). It occurs as foliated to fibrous masses, and in an exceptionally rare crystal form. It has a perfect basal cleavage, and the folia are non-elastic, although slightly flexible. It is the softest known mineral and listed as ''1'' on the Mohs hardness scale. As such, it can be easily scratched by a fingernail. It has a specific gravity of 2.5–2.8, a clear or dusty luster, and is translucent to opaque. Talc is not soluble in water, but it is slightly soluble in dilute mineral acids. Its color ranges from white to grey or green and it has a distinctly greasy feel. Its streak is white. Soapstone is a metamorphic rock composed predominantly of talc. ==Formation== Talc is a metamorphic mineral resulting from the metamorphism of magnesian minerals such as serpentine, pyroxene, amphibole, olivine, in the presence of carbon dioxide and water. This is known as ''talc carbonation'' or ''steatization'' and produces a suite of rocks known as talc carbonates. Talc is primarily formed via hydration and carbonation via the following reaction; :''serpentine + carbon dioxide → talc + magnesite + water'' :2 Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3 MgCO3 + 3 H2O Talc can also be formed via a reaction between dolomite and silica, which is typical of skarnification of dolomites via silica-flooding in contact metamorphic aureoles; :''dolomite + silica + water → talc + calcite + carbon dioxide'' :3 CaMg(CO3)2 + 4 SiO2 + H2O → Mg3Si4O10(OH)2 + 3 CaCO3 + 3 CO2 Talc can also be formed from magnesian chlorite and quartz in blueschist and eclogite metamorphism via the following metamorphic reaction: :''chlorite + quartz → kyanite + talc + water'' In this reaction, the ratio of talc and kyanite is dependent on aluminium content with more aluminous rocks favoring production of kyanite. This is typically associated with high-pressure, low-temperature minerals such as phengite, garnet, glaucophane within the lower blueschist facies. Such rocks are typically white, friable, and fibrous, and are known as whiteschist. Talc is a tri-octahedral layered mineral; its structure is similar to that of pyrophyllite, but with magnesium in the octahedral sites of the composite layers.〔 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「talc」の詳細全文を読む スポンサード リンク
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