Crystal system: hexagonal. Cleavage: frequently rhombohedral crystals with curved faces, similar to dolomite. Color: light to dark brown. Hardness: 3.5-4. Luster: vitreous. Specific gravity: 3.96. Acid test: soluble in hot hydrochloric acid.
Siderite is a common mineral associated with geodes, nodules, and ironstone banding. Siderite is very common in the coal fields, where it occurs as a reddish-brown mineral in shale layers, nodules, concretions, and fossil burrows. Commonly, a siderite nodule will have a nucleus of another mineral such as pyrite, sphalerite, or chert. Siderite also occurs in geodes with dolomite and calcite. Siderite becomes magnetic on heating, and alters readily to limonite and goethite.
Siderite and associated iron minerals form curved bands common in sandstones in Kentucky. These bands are called Liesegang bands.
In the Mazon Creek area of Illinois, siderite nodules that look similar to siderite nodules found in the coal fields of Kentucky are found. Do you know what is found in Mazon Creek siderite nodules? Click on the image below to see inside these nodules.
Siderite nodules from Mazon Creek are world-famous for the fossils they sometimes contain. Fossils of plants, like the fern on the left are most common. But fossils of soft-bodied organisms, like the millipede on the right, are the most famous. Can you see the legs on the milllipede? Some brachiopod and clam fossils have been found in siderite nodules in Kentucky, but no fossils of soft-bodied organisms have been found.