In nature it is frequently altered to limonite with the separation of native sulphur.
They are four important iron ores, magnetite, haematite, limonite and siderite, and one of less but still considerable importance, pyrite or pyrites.
C. Ullmann's name of stilpnosiderite, from the Greek ariAirvOs (shining) is sometimes applied to such kinds of limonite as have a pitchy lustre.
Goethite, limonite and haematite are found in New South Wales, at the junction of the Hawkesbury sandstone formation and the Wianamatta shale, near Nattai, and are enhanced in their value by their proximity to coal-beds.
Gothite occurs with other iron oxides, especially limonite and hematite, and when found in sufficient quantity is mined with these as an ore of iron.
The brownish colour of some slates is due to limonite and haematite, but magnetite occurs in the darker coloured varieties.
Deposits of limonite in cavities may have a rounded surface or even a stalactitic form, and may present a brilliant lustre, of blackish colour, forming what is called in Germany Glaskopf (glass head).
The various kinds of brown and yellow ochre are mixtures of limonite with clay and other impurities; whilst in umber much manganese oxide is present.
When limonite is dehydrated and deoxidized in the presence of carbonic acid, it may give rise to chalybite.
Iron is widely diffused, principally in the form of magnetite, brown haematite, limonite and bog iron.