Note: Do not rely on this information. It is very old.
Talc, an Arabic name applied properly to a hydrated magnesium silicate, usually found in a massive or foliated, non-crystalline form. The folia are flexible, but not elastic, thus differing from those of mica, with which talc is popularly confused. Talc does, however, rarely occur in crystals belonging to the Prismatic system. It is white or greenish, translucent, pearly, sectile, and greasy. Its hardness is 1 to 1.5, and its specific gravity is 2.5 to 2.8. It is not decomposed by acids, and before the blowpipe becomes opaque and exfoliates. Steatite, soapstone, and French chalk are merely amorphous unfoliated varieties. Talc is undoubtedly a decomposition-product of various anhydrous magnesium silicates.