Chloritoid

specimen number:
2062144

location:
Champion Mine, Marquette Co., Michigan, USA

description:
Rich masses of chloritoid throughout matrix. Locally this material is called masonite. Nice example for the species.

Chloritoid is a manganese iron aluminum silicate hydroxide mineral that typically forms during low-to-medium grade metamorphism of aluminum-rich sedimentary rocks like pelites. Geologically, it is often found in phyllites, schists, and marbles, where it serves as a significant index mineral for specific pressure and temperature conditions within the greenschist facies. The crystal structure of chloritoid is characterized by a monoclinic or triclinic symmetry, often appearing as platy, micaceous masses or distinct pseudo-hexagonal crystals with a dark green to blackish-gray color. Unlike the true micas, chloritoid is notably brittle and has a higher hardness, usually ranging between 6 and 7 on the Mohs scale, which helps distinguish it in hand samples. Its presence is a key indicator of the metamorphic history of a region, frequently occurring alongside other minerals such as muscovite, chlorite, and quartz.

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