Haematite. Little Broken Hill, Australia
A highly metamorphosed iron formation. Coarsely crystalline haematite (white) has totally replaced magnetite and so is martite. The poorly polished cores (centre top) show less complete replacement than the well polished rims. The matrix comprises a mosaic of equigranular garnet (light grey, centre bottom) and quartz (dark grey) with characteristic 1200 angles between adjacent crystals. Black areas are polishing pits.
Polished block, plane polarized light, x 40, air