Vivianit

Vivianit is a hydrated iron phosphate mineral with the chemical formula Fe3(PO4)2·8H2O. It forms as a secondary precipitate in phosphate-rich, reducing environments such as certain sedimentary settings, bogs, and groundwater deposits. It is often found in association with minerals like siderite, pyrite, goethite, calcite, and apatite.

In terms of appearance, vivianit usually occurs as blue, greenish-blue, or colorless crystals and aggregates. Freshly prepared or recently exposed material tends to be pale blue, and the color can deepen with exposure to air and light due to changes in iron oxidation state. The mineral has a vitreous to pearly luster and ranges from transparent to translucent. Vivianit crystallizes in the monoclinic system and commonly forms prismatic crystals or radiating groups. It has a Mohs hardness of about 1.5–2 and a relatively low density around 1.8–1.9 g/cm³.

Vivianit is not a major ore of iron or phosphate because of its hydrates, softness, and limited abundance, but it is of interest to mineral collectors and researchers studying phosphate-bearing diagenetic and depositional environments. Its relatively simple hydration state makes it sensitive to dehydration and oxidation, which can alter it to other iron phosphate phases.

Occurrences are reported worldwide in phosphate-rich sedimentary contexts, including boggy, peat-rich areas and reduced groundwater systems. Notable finds have come from Europe and North America, among other regions, reflecting its preference for specific geochemical conditions rather than widespread abundance.