C—colorless, H—2-2.5 In scales or “books”; in Muscovite silver white, Cl—perfect in splits into thin sheets; (white mica) gray, brown one direction common in sandstones, (23) shales, and in igneous (OH)₂KAl₂ and metamorphic rocks AlSi₃O₈ C—brown or black H—2.5-3 As scales or “books”; Biotite (23) Cl—perfect in splits into thin sheets; (black mica) one direction common in igneous and (OH)₂K(Mg,Fe)₃ metamorphic rocks but AlSi₃O₈ not in sedimentary rocks such as sandstone or shale C—colorless, H—3 Common mineral; Calcite (24) white, gray, Cl—perfect in effervesces vigorously CaCO₃ and various three in cold acid; occurs in tints directions, not many crystal forms and at right angles as fibrous, banded, and (rhombohedral) compact masses; chief mineral in limestones C—white, gray, H—3 Very heavy; commonly in Barite red, or almost Cl—perfect in tabular crystals united BaSO₄ any color one direction, in diverging groups, as less perfect in laminated or granular two other masses; associated with directions fluorite in southern Illinois C. Hardness greater than 3 but not greater than 5 (Cannot be scratched by penny; can be scratched by knife)