Talc

Talc, steatite and soapstone are the names used in trade circle for variations in geological
mode of occurrence and in purity, of the same mineral the original name of which is talc (the
word has been adopted in English from the Arabic name “talq”). It is a isomorph of
pyrophyllite (i.e. they share the same monoclinic structure but have different chemistries).
The physical properties of the two minerals as well as their uses bear many similarities. Pure
talc is a hydrated silicate of magnesium (3MgO.4SiO2.H2O) theoretically containing 63.5%
SiO2, 31.7% MgO and 4.8% H2O and is characterized by extreme softness (cf., composition
of pyrophyllite is Al2O3.4SiO2.H2O). It is generally found in nature in massive form. The
nomenclature steatite is now generally restricted to the massive compact crypto-crystalline
variety of high grade talc, while the name soapstone refers to the massive impure talcose rock
generally containing 50-80% talc mixed with chlorite, amphibole, pyroxene, mica, pyrite,
quartz, calcite, dolomite etc. Talc may occur in the form of flakes and fibers or blocks. The
latter is sometimes referred to as “block talc” or “lava” (general name for any talc or
soapstone) or “block steatite talc” (specific name for steatite variety of talc occurring in block
form) or “french chalk” (a very soft massive variety of talc).
Talc occurs in chloritic schist, gneiss and dolomite. It is generally derived from either
magnesium-bearing sedimentary rocks or ultrabasic igneous rocks, the former type being
purer than the latter. It is formed in the former case by metamorphism under conditions of
high temperature and/or pressure due to geological disturbance, igneous intrusion, etc. and in
the latter case by hydrothermal alteration of non-aluminous magnesium silicates (e.g.,
asbestos, amphibole, pyroxene, etc.).