hiilikite

Hiilikite is a mineral species described in geological literature as a carbon-rich, black mineral commonly found in coal measures. The name is derived from the Finnish word hiili for coal, combined with the standard mineral suffix -ite.

Chemically, hiilikite is interpreted as a mainly carbonaceous phase with minor inorganic components such as silicon, iron, and calcium. It occurs as microcrystalline masses or nodules within coal seams and metamorphosed organic-rich rocks. Its crystalline structure ranges from poorly defined to microcrystalline, and it often lacks a uniform chemical formula, reflecting variable composition.

Occurrence and formation: Hiilikite has been reported from several coal-bearing basins and similar sedimentary environments. It is thought to form through diagenetic alteration of peat and plant matter during burial, with conditions that promote partial graphitization and restructuring of organic matter under low-grade metamorphism. In some specimens, it is associated with graphitic carbon and other carbon-rich minerals such as pyrite and calcite.

Physical properties and classification: Hiilikite typically presents as black to dark gray, with a metallic to submetallic luster. Mohs hardness is commonly 2–4, and the density ranges approximately from 2.2 to 3.0 g/cm3. It is commonly described as a native carbonaceous mineral and is of limited industrial use, though it can be of interest in petrographic studies and discussions of the carbon cycle.

See also: carbon minerals, carbon-rich metamorphic rocks, coal geology.