Also found in: Wikipedia.


FeAl4Si2O10(OH)4 A micaceous mineral related to the brittle mica group; has both monoclinic and triclinic modifications, a gray to green color, and weakly pleochroic crystals.



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.

References in periodicals archive ?
Mica schist have a paragenesis of quartz + muscovite + biotite with minor garnet, chloritoid and plagioclase, and locally andalusite porphyroblasts cutting the [S.
24 78 12 7 2 1 Chloritoid analyses and structural formulae (based on 12 oxygen) S68-65 24.
Much of the andalusite and almost all staurolite in these rocks has been retrograded to pleochroic blue-green needles of chloritoid with Fe/Fe+Mg = 0.