L361 - Oolitic ironstone

This rock is made up of light grey sub-spherical ooids. 
Fine to medium sand grain size. 
A few lithic/biogenic fragments. 
Dark grey cement. 
Small hair-line white veins run through this rock.

Ooids made up of a greenish sheet silicate mineral called chamosite, partially altered to siderite and limonite. Concentric structure. Central cores of fine-grained material.

A layer formed of brown, transparent crystals, sometimes with rhombic cross-sections surrounds the peripheries of many ooids. This is siderite.

Some quartz grains.

The cement is made up mainly of carbonate (siderite), with a small quantity of a green-brown material, which has anomalous interference colours.

This rock is a grainstone (Dunham naming system) or oo-sparite (Folk naming system).

Deposited in a moderate energy, marine energy.

Chamosite is an Fe-rich chlorite, often greenish. It is a sheet silicate.

Limonite is amorphous or cryptocrystalline, and yellow-brown to brownish-black in colour. It comprises hydrated iron (III) oxides with a range of compositions.

Siderite is iron carbonate (FeCO₃). It is often difficult to distinguish siderite from calcite and dolomite optically.

oolitic ironstone
oolite