Haematite, chamosite and limonite. Egypt
Ooids with a central nucleus of chamosite (dark grey) have concentric outer bands of haematite (white, highest reflectance) where the individual haematite crystals lie tangentially. Slight differences in reflectance (centre) are due to variations in the grain size of the haematite between the bands, or to the ratios of haematite, limonite and silicates within the bands. Limonite (light grey, right) is banded; it forms the matrix and so infills voids between the ooids. Dark grey areas are chamosite (top left). Black areas are voids.
Polished block, plane polarized light, x 40, air