Clay ironstones and oolitic iron ores




Siderite, pyrite and marcasite. Great Oakley, Britain


Click hereA vug within a siderite nodule is rimmed by a fine-grained intergrowth of pyrite and marcasite which cannot be distinguished from each other at this magnification. Bladed marcasite (centre bottom) grows from this rim into the vug. The matrix is a mosaic of siderite crystals which show faint bireflectance from grey-blue to grey-brown (top left).



Polished block, plane polarized light, x 80, air


Siderite, limonite and pyrite. Great Oakley, Britain


Click hereA section through the core (right) to rim (left) of a clay ironstone nodule. The core of the nodule comprises fine-grained siderite crystals (light grey, right) which show faint bireflectance. A thin zone of coarse-grained carbonate (light grey, centre) shows bireflectance and lies between siderite and limonite (blue-grey, higher reflectance, centre and left), which has replaced the siderite extensively. The outer surface of the nodule (top left) is very weathered and all carbonate has been removed. A small pyrite framboid (yellow-white, centre left) lies within the limonite.



Polished block, plane polarized light, x 80, air


Haematite, chamosite and limonite. Egypt


Click hereA 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


Haematite, chamosite and limonite. Egypt


Click hereHaematite (blue-white, highest reflectance) forms the central ooid, which has a poorly defined concentric banding. Haematite is present in other ooids, again following the concentric banding. Limonite (light grey, left top, right centre, bottom centre) is banded and incompletely infills voids (black) between the ooids, so forming the matrix. Dark grey areas are chamosite (top right).



Polished block, plane polarized light, x 40, air


Haematite, chamosite and limonite. Egypt


Click hereOoids 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


Haematite, chamosite, limonite and TiO2 mineral. Egypt


Click hereAn irregular-shaped chamosite core (green internal reflections, right) is surrounded by haematite (blue-white) bands where individual haematite crystals lie along the bands. Limonite (blue-grey, top centre) shows faint orange internal reflections which are stronger in smaller limonite crystals (top left). An angular clast of a TiO2 mineral (white, centre left) has higher reflectance than haematite. Chamosite ooids (bottom left, top centre) have concentric bands of chamosite (yellow-green internal reflections). In thin section, most of the ooids show pseudo-uniaxial crosses which suggest that their chamosite crystals are radially oriented. The matrix is limonite and limonitically-stained chamosite (brown chamosite in thin section).



Polished block. plane polarized light, x 160, oil