A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5400 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
The oxide minerals all contain one or more oxide anions (O-2). The oxide minerals include species that are hydroxy-oxides. The hydroxide minerals (those with one or more OH-) are usually considered together with the oxides. Many sulfide minerals are not stable in Earth-surface conditions. In the presence of oxygen and moisture, sulfide minerals tend to tarnish or alter to oxides and hydroxy-oxides. All except the most inert elements (such as the platinum-group elements and gold and noble gases) readily form oxides. Gold oxide forms only under special conditions.
The exact nature of the material called turgite is not agreed upon. Some mineralogists consider turgite to be a mixture of hematite and goethite that results from goethite alteration. Others regard turgite as a hydrous hematite mineral (2Fe2O3·H2O) or as "limonite". Regardless, turgite is fascinating and beautiful material. Turgite often occurs as rainbow-colored iridescent coatings on iron oxide-rich rocks or rocks having surficial iron oxide staining. It can also occur as irregularly botryoidal masses.
Locality: Elba, offshore from the western coast of Italy
Photo gallery of turgite: