Talc

[|Soapstone] is a [|metamorphic rock] composed predominantly of talc.
 * Talc** (derived from the [|Persian] //tālk// (تالک ), [|Arabic] //talk// (تلك)) 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 talcum powder. It occurs as foliated to fibrous masses, its [|crystals] being so rare as to be almost unknown. 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] . It can be easily scratched by a [|fingernail] . It is also [|sectile] (can be cut with a knife). 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 colour ranges from [|white] to [|grey] or [|green] and it has a distinctly greasy feel. Its [|streak] is white.

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. [|[2]]