talc talc talc, or talcum, is a clay mineral, composed of hydrated magnesium silicate with the chemical formula mg3si4o10(oh)2. talc in powdered form, often combined with corn starch, is used as baby powder. this mineral is used as a thickening agent and lubricant. it is an ingredient in ceramics, paints, and roofing material. it is a main ingredient in many cosmetics. it occurs as foliated to fibrous masses, and in an exceptionally rare crystal form. it has a perfect basal cleavage and an uneven flat fracture, and it is foliated with a two-dimensional platy form. talcgeneralcategorysilicate mineralformula(repeating unit)mg3si4o10(oh)2ima symboltlcstrunz classification9.ec.05crystal systemmonoclinic or tricliniccrystal classeither prismatic (2m) or pinacoidal (1)space groupc2/c or c1unit cella = 5.291 å, b = 9.173 åc = 5.290 å; α = 98.68°β = 119.90°, γ = 90.09°; z = 2 ora = 5.287 å, b = 9.158 åc = 18.95 , β = 99.3°; z = 4identificationcolorlight to dark green, brown, white, grey, colorlesscrystal habitfoliated to fibrous masses, rare as platey to pyramidal crystalscleavageperfect on basal cleavagefractureflat surfaces (not cleavage), fracture in an uneven patterntenacitysectilemohs scale hardness1 (defining mineral)lusterwaxlike or pearlystreakwhite jot to pearl blackdiaphaneitytranslucentspecific gravity2.58 to 2.83optical propertiesbiaxial (-)refractive indexnα = 1.538 – 1.550nβ = 1.589 – 1.594nγ = 1.589 – 1.600birefringenceδ = 0.051pleochroismweak in dark varietiesultraviolet fluorescenceshort uv=orange yellow, long uv=yellow