vivianiit

Vivianiite, also known as vivianite in English, is a hydrated iron phosphate mineral with the chemical formula Fe3(PO4)2·8H2O. It typically forms blue to blue-green crystals or masses and is notable for its tendency to darken when exposed to air. The mineral crystallizes in the monoclinic system and has a vitreous luster, with transparency ranging from transparent to translucent. Its Mohs hardness is about 2, and its streak is pale grey.

Occurrence and formation

Vivianiite forms in reducing, phosphate-rich environments, such as marine and freshwater sediments, bogs, and diagenetic nodules. It is a secondary mineral that can crystallize when iron, phosphate, and water are present under anoxic conditions. In the geological record, it is often associated with other phosphates as well as minerals like siderite, pyrite, and calcite.

Properties and behavior

In nature, vivianiite is delicate and can be easily scratched or altered. When exposed to air, the iron in vivianiite oxidizes from Fe2+ toward Fe3+, causing a color change from blue to greenish-brown and sometimes resulting in a whitish or brownish alteration rim. It can dehydrate under certain conditions, gradually losing water of crystallization.

Uses and occurrence in collections

Vivianiite is primarily of interest to mineral collectors and researchers studying phosphate-rich diagenetic environments. It is not a major ore mineral due to its softness and stability challenges.

See also

List of minerals, phosphate minerals, diagenesis.