Talc Talc Talc or talcum is a clay mineral composed of hydrated magnesium silicate with the chemical formula HMg(SiO) or MgSiO(OH). In loose form, it is (in ratio with or without corn starch) one of the most widely used substances known as baby powder. It occurs as foliated to fibrous masses, and in an exceptionally rare crystal form. It has a perfect basal cleavage, and the folia are not elastic, although slightly flexible. The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 1 as the hardness of talc. Talc can easily be scratched by a fingernail. Talc
Bursaite hand samples of bursaite appear to be nearly identical to the mineral lillianite. In a microscope, bursauite shows distinct high reflective power and oblique extinction. The reflectance values are Rγ' = ~43 and Rα' = ~38 (in nm). The birefringence is generally weak in air, but stronger when bursaite is immersed in oils. Bursaite has a hardness of 2.5 on the Mohs scale. It has a gray, metallic luster that appears white in polished sections. The mineral is also known to have good tabular cleavage along the (100) planes. Like many sulfosalts, bursaite occurs in regions abundant in sulfur. Close