Habit: Brown, yellow or white. Crystals rhombohedral with curved or saddle-shaped faces, prismatic to tabular or pseudo-octahedral columnar; also stalactitic, granular or massive. Vitreous to pearly luster; translucent to transparent. White streak.
Environment: Formed in low-grade metamorphosed ironstones and sedimentary banded iron formations, in carbonatites or as a product of hydrothermal alteration of carbonate sediments, and in hydrothermal sulfide veins.
Etymology: Named after Professor Matthias Joseph Anker (1772-1843), an Austrian mineralogist.