Formation
(1) Primarily through precipitation from hydrothermal solutions at low temperatures (< 240 °C) and acidic pH values (pH < 5) under reducing conditions.
(2) Syngenetic as a diagenetic formation in reducing, organic-rich sediments (e.g., coal, clays) from sulfide-containing pore waters.
(3) Secondary as a conversion product of other iron sulfides (especially magnetite) under near-surface, mostly acidic conditions.
