Iron carbonate (FeCO3), or siderite from the Greek sideros meaning iron, is a major source of ore for steel-making being usually low in sulphur and phosphorous, and high in manganese and/or magnesium.
This mineral is able to assume almost any colour but commonly brown, yellowish-brown, or grey specimens can be found. It occurs in Britain’s Carboniferous strata as nodules and beds of impure iron carbonate known as Clay Ironstone. Once a valuable source of ore, alongside a dark carbonaceous form known as Blackband. In Cleveland the well-known ironstone through which which Teesside became a major industrial force from 1850, is of Jurassic age (c.188,000,000 years old), contains iron-rich berthierene rather than siderite, and occurs with a distinctive texture known as oolitic. An amalgamation of small rounded concentric structures, which form through the same colloidal processes as those reponsible for oolitic limestones, make up the bulk of the rock. Siderite can also be found in massive, granular, or concretionary forms, produced in a variety of environments including within hydrothermal veins along with pyrite and galena, within intrusive pegmatites, and as sedimentary Bog Iron Ore in high latitude lakes and swamps.
At its purest, siderite forms rhombohedral crystals with a vitreous (inclining to pearly) lustre, perfect cleavage, a white streak, and uneven fracture. An allied mineral Hydrated Iron Oxide or Limonite (FeO(OH)·nH2O), commonly forms pseudomorphs (perfect copies) of siderite crystals.
It is however more usually found in the local area as red-weathering nodules within grey mudstone scars, exposing part of the Cleveland Ironstone Formation that crops out on the foreshore at Jet Wyke, Staithes. In the 1700s, such nodules were collected from the scars by local villagers and loaded onto boats which eventually disgorged their cargoes at furnaces on Tyneside, long before the significance of the Cleveland Ironstone Formation was suspected.
Images above are of:
Manganoan Siderite with Albite;
Siderite Pseudomorphosis in Limonite with quartz;
Red Siderite Nodule in Grey Mudstone at Staithes.