Limonite, FeO OH n H2O, is used mainly as a field term to refer to natural hydrous iron oxides of uncertain identity. It is a common and widely distributed mineral. For photographs of each mineral type and detailed descriptions of this mineral family please visit the Mineral Gallery . Minerals are indexed by name, group, and interest. Fragments of calcite effervesce readily in cold dilute HCl.
Calcite is one of the most common and widespread minerals.
Crystalline, metamorphosed limestones are marbles. The most important use for calcite is for the manufacture of cements and lime for mortars. Quicklime when mixed with sand forms common mortar. Small amounts of magnesium carbonate and iron oxide are also present. Limestone is a raw material for the chemical industry, and a clear variety known as Iceland spar is used in optical instruments.