Chloritoid

a mineral of the nesosilicate class, with a chemical composition of (Fe, Mg)₂(Al, Fe³⁺)Al₃[SiO₄]₂O₂(OH)₄. Chloritoid containing up to 16.6 percent MnO is called ottrelite.

Chloritoid crystallizes in the monoclinic or triclinic system; both modifications are equally common and often form dense concretions. The mineral usually occurs in the form of poorly developed tabular crystals and concretions (chloritoid porphyroblasts in metamorphosed schists) and in the form of plumose aggregates. Generally, its color is dark blue or dark green, sometimes ranging to nearly black. Chloritoid has a hardness of 6.5 on Mohs’ scale and a density of about 3,600 kg/m³. It is brittle and exhibits perfect cleavage parallel to the flattening of the crystals. A rock-forming mineral in certain metamorphosed schists and contact hornfels, chloritoid is also found in deposits of emery, in hydrothermal quartz veins, and in the rocks surrounding veins.