A hydrated magnesium layer silicate (phyllosilicate) with composition close to Mg₃Si₄O₁₀(OH)₂. Limited substitution of aluminum (Al) or titanium (Ti) for silicon (Si) and of iron (Fe), manganese (Mn), or aluminum for magnesium (Mg) is possible. Steatite is massive talc aggregate that contains less than 1.5% calcium oxide (CaO), 1.5% iron(II) oxide + iron(III) oxide (FeO + Fe₂O₃), and 4% aluminum oxide (Al₂O₃). Talc commonly is white, but it may appear pale green or grayish depending on the amount of minor impurities. It usually occurs in coarse to fine platy or fibrous aggregates that often have a more or less parallel arrangement. Shreds and plates are often bent; some tabular crystals exhibit perfect basal cleavage, yielding flexible, slightly elastic lamellae. Talc has a greasy feel and pearly luster, and has been used as one of the hardness standards for rock-forming minerals with the value 1 on the Mohs scale. Because talc is soft, it can be scratched by fingernails.