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.

Use of this mineral can be traced back to c.3,000BC, when the Ancient Egyptians used it as a kind of cosmetic known as Kohl applied around the eyes. The dark marks it produces played the dual role of both reducing the glare of the powerful desert sun, and also as a fly repellent which helped reduce the instance of disease. Though it was used in this manner by the Egyptians, being an ore of lead it is also harmful to health causing lead poisoning. Galena is also a semiconductor (like silicon in today’s computers) and was used to detect radio signals in early receivers as a point-contact diode.

Mining for lead in Britain probably pre-dates the Roman occupation, but it was during their tenure here that the mineral appears to have first been extracted on a large scale. In many places small areas of Roman workings can still be found today. The Romans located the ore by looking for a small white flower, Spring Sandwort (Minuartia verna), also known as Leadwort; This flower grows on land heavily polluted by lead and other associated heavy metals. Where the Romans noticed this plant growing they would scrape back the soil and grass to locate the vein or rake of lead which was then extracted and smelted.

Lead is often found in hydrothermal veins and is associated with silver, pyrite, calcite, fluorspar and barites, all of which are useful by products known as gangue minerals.

The images above show a large boulder with Galena, the flower Spring Sandwort, and a fist-sized sample of Galena with flourite.