Metavivianite

Chemical formula: Fe²⁺Fe³⁺₂(PO₄)₂(OH)₂·₆H₂O

Metavivianite is a rare, secondary phosphate mineral characterized by its dark blue or greenish-black color, formed by the oxidation of vivianite.

## Characteristics Metavivianite is a hydrated iron phosphate, closely related to vivianite. It forms as a product of vivianite's oxidation, which leads to the conversion of some iron (II) to iron (III). Typical specimens consist of aggregates of small, bladed or fibrous crystals, often forming coatings and crusts on other minerals or in rock fissures. Its color is intense, ranging from dark blue to bluish-black or from dark green to greenish-black. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 1.5-2, meaning it can be scratched with a fingernail. It is translucent and has a luster described as vitreous to dull, depending on its form of occurrence. Due to its brittleness and small crystal size, it is a delicate mineral. ## Colors and Varieties Metavivianite occurs in shades of dark blue, navy blue, and also dark green, often transitioning to almost black. Color variations result from the ratio of divalent and trivalent iron in its structure. No named commercial or color varieties are distinguished. ## History and Name The name "metavivianite" refers to its close relationship with vivianite and the fact that it is a product of its alteration (Greek "meta" - after, alongside). The mineral was formally described and recognized in 1974. Its type locality is the Big Chief Mine in Pennington County, South Dakota, USA. ## Uses Metavivianite has no industrial application. Its significance is purely scientific and as a collector's item, serving as an interesting example of a secondary mineral and a member of the vivianite group.

Properties

Mohs hardness: 1.5-2
Luster: Vitreous to dull
Streak: Blue or greenish blue
Density: 0
Cleavage: {110}
Transparency: Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Metavivianite can be identified by its characteristic dark blue or greenish-black color, very low hardness (1.5-2 on the Mohs scale), and occurrence as small, bladed or fibrous crystals. It often forms coatings and crusts in paragenesis with vivianite and other phosphates. ## Distinguishing from Similar Minerals It is most commonly confused with vivianite, from which it originates. Fresh vivianite is colorless or pale blue and darkens upon exposure to light, whereas metavivianite is by definition a dark mineral. Definitive differentiation requires advanced analytical methods (e.g., XRD). It can also be confused with other dark secondary minerals, such as ludlamite or rockbridgeite, from which it is distinguished by lower hardness and specific crystal habit. ## Crystal Forms Metavivianite crystals are usually very small, with a bladed or acicular habit. They most often form radial or tangled aggregates, as well as fibrous and earthy masses. Well-formed, isolated crystals are rare.

Geological environment

## Genesis Metavivianite is a secondary mineral. It forms in the oxidation zone of deposits containing phosphate minerals, as a result of the alteration (oxidation) of previously formed vivianite. This process occurs under the influence of atmospheric oxygen, leading to a change in the oxidation state of iron in the crystal structure. ## Mineral Associations It co-occurs with minerals in whose company it forms. Most commonly, these are vivianite (as the parent mineral), as well as other secondary iron and manganese phosphates, such as ludlamite, rockbridgeite, strengite, or hureaulite. Goethite and other iron oxides may also accompany it. ## Localities The most important occurrences of this rare mineral are in the USA, including its type locality – the Big Chief Mine in South Dakota. It has also been reported in mines in New Hampshire. Outside the USA, its occurrences have been confirmed in Cameroon (N'gaoundéré area), Japan (Ashio mine, Tochigi Prefecture), and Bolivia (mines in the Potosí department), among others.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, even if small, crystals of intense, deep blue color are most valued by collectors. Contrastive placement on a light rock matrix enhances their attractiveness. Due to the nature of the mineral, even rich coatings and fibrous aggregates are considered valuable if they have a vivid color and are not contaminated. ## Popular Localities Specimens from the type locality in South Dakota (USA) and from Bolivian mines (e.g., Siglo XX, Llallagua) are the most well-known among collectors. Japanese specimens from the Ashio mine are also considered classic material for this species.

Care and storage

## Cleaning Metavivianite specimens are extremely delicate and sensitive. Cleaning should be limited to an absolute minimum. If necessary, a very soft brush can be used to remove loose dust. Avoid water and any chemical agents. ## What to Avoid The mineral is very soft and brittle, so any physical contact, ultrasonics, and mechanical cleaning should be avoided. It is sensitive to changes in humidity and temperature. It should be protected from direct sunlight, which can cause further chemical changes and specimen degradation. ## Storage It is recommended to store specimens in enclosed, stable conditions, preferably in "micromount" boxes or display cases that protect against dust, light, and sudden changes in humidity. Avoid storage in areas with high humidity.