Haematite, chamosite and limonite. Egypt
A number of round or discoidal ooids comprise concentric bands of fine-grained haematite, limonite and chamosite. The central ooid has a nucleus of banded haematite (white-grey, higher reflectance) which may be a fragment of an earlier ooid. This nucleus is surrounded by concentric bands of chamosite (dark grey, lower reflectance), haematite and, in the outer zones, limonite (medium grey, lower reflectance than haematite). Rhythmically banded limonite (grey, right centre, top centre) is associated with voids. Chamosite (dark grey) is the main matrix between the ooids.
Polished block, plane polarized light, x 40, air