Mantienneite

Chemical formula: (H<sub>2</sub>O)<sub>2</sub>Mg<sub>2</sub>(Al<sub>2</sub>Ti<sup>4+</sup>)(PO<sub>4</sub>)<sub>4</sub>(OH)<sub>2</sub>(H<sub>2</sub>O)<sub>10</sub>·4H<sub>2</sub>O

Mantienneite is a very rare, hydrated magnesium, aluminum, and titanium phosphate, forming characteristic pearly aggregates of bladed crystals.

## Characteristics Mantienneite is a mineral from the phosphate group, chemically classified as a hydrated magnesium, aluminum, and titanium phosphate. It occurs as small aggregates and rosettes, composed of thin, bladed or tabular crystals. Individual crystals are usually very small, rarely exceeding a few millimeters in length. The aggregates have a characteristic radial or fan-like arrangement. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2. It is brittle and has perfect cleavage in one direction, which causes it to easily split into thin lamellae. On cleavage surfaces, it exhibits a pearly luster, while on other surfaces, it is vitreous. Its density is low, approximately 2.25 g/cm³. It is transparent to translucent. ## Colors and Varieties Mantienneite is usually colorless or white. No colored varieties or trade names are known. ## History and Name The mineral was first described in 1977 by A. M. Fransolet, K. Abraham, and W. Schreyer. The name honors Jean Mantienne (1916–1975), a French geologist who studied pegmatites in Cameroon, including the one where the discovery was made. The type locality is the Anloua pegmatite in Cameroon. ## Uses Due to its extreme rarity and small crystal size, mantienneite has no industrial applications. It is solely an object of scientific and collecting interest for specialists who collect rare minerals.

Properties

Mohs hardness
2
Luster
Pearly, Vitreous
Streak
White
Density
2.25
Cleavage
Perfect on {010}
Transparency
Transparent to Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Characteristic features of mantienneite include its specific form of occurrence as rosettes composed of bladed crystals, very low hardness (2 on the Mohs scale), and pearly luster on cleavage surfaces. Its unique environment of occurrence and mineral paragenesis are also crucial for identification. ## Distinguishing from Similar Minerals Mantienneite can be confused with other white, secondary phosphate minerals with a similar habit, such as wavellite. However, wavellite usually forms spherical, radial aggregates, not flat rosettes. It can also be mistaken for some zeolites or talc, but it differs from them in chemical composition and genesis. Certain differentiation most often requires advanced studies, e.g., chemical analysis (EDS) or X-ray diffraction (XRD). ## Crystal Forms It forms thin, flattened, bladed crystals, elongated in one direction. These crystals almost always occur in radial or fan-shaped aggregates and rosettes.

Geological environment

## Genesis Mantienneite is a secondary mineral. It forms in the oxidation zone of granitic pegmatites as a result of low-temperature hydrothermal processes and weathering. It forms in rock cavities and fractures due to the alteration of primary phosphate minerals (e.g., triplite) and beryl. ## Mineral Associations It co-occurs with other secondary phosphates, such as wavellite, crandallite, augelite, and variscite. ## Localities The only confirmed and well-described locality of mantienneite in the world is its type locality – the Anloua pegmatite, located near Nanga-Eboko in central Cameroon.

Rarity

Very rare

For collectors

## Quality Criteria The quality of mantienneite specimens is assessed primarily based on the richness and aesthetics of the aggregates. Specimens with well-formed, sharp crystals forming distinct rosettes or fans on a small rock matrix are most valued. The size of the aggregates and the absence of mechanical damage are crucial. Due to its rarity, even microscopic but well-formed specimens are valuable for specialized collectors. ## Popular Localities All collectible specimens come from a single location in the world – the Anloua pegmatite in Cameroon, which makes this mineral one of the most geographically restricted.

Care and storage

## Cleaning Mantienneite is an extremely delicate, soft, and brittle mineral. It should only be cleaned dry, using a very soft brush to remove dust or carefully using compressed air from a safe distance. Contact with water is not recommended due to its hydrated structure. ## What to Avoid Ultrasonic cleaners, all chemical agents, acids, and detergents should be absolutely avoided. The mineral is sensitive to shocks and impacts due to its perfect cleavage. Heating can lead to its dehydration and destruction of its structure. ## Storage Mantienneite specimens should be stored in separate, padded boxes (micromount type) to protect them from mechanical damage and contact with harder minerals. Stable temperature and humidity conditions should be ensured.

Sources

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