Mengeite

Chemical formula: Ba(Mg,Mn<sup>2+</sup>)Mn<sup>3+</sup><sub>4</sub>(PO<sub>4</sub>)<sub>4</sub>(OH)<sub>4</sub>·4H<sub>2</sub>O

Mengeite is a very rare, hydrated barium, magnesium, and manganese phosphate, forming microscopic, dark brown to black tabular crystals.

## Characteristics Mengeite is an extremely rare mineral from the phosphate group, chemically classified as a hydrated barium, magnesium, and manganese phosphate with varying oxidation states. It occurs as very small, tabular or bladed crystals, whose size rarely exceeds fractions of a millimeter. It typically forms small, radial aggregates or rosettes. Its color ranges from dark brown to almost black, which, combined with the small crystal size, makes it difficult to observe and identify with the naked eye. ## Physical Properties Mengeite crystals exhibit a luster described as vitreous to metallic. They are brittle and opaque. Due to the microscopic size of the crystals, accurately determining many physical properties, such as hardness or density, is very difficult and has not been precisely established. ## History and Name The mineral was named in honor of John Menge (1788–1852), a German geologist and explorer who first discovered phosphates in the Angaston area of South Australia, where this mineral was later identified. It was formally described and approved as a new mineral species by the International Mineralogical Association (IMA) in 2012. ## Uses Due to its extreme rarity and microscopic size, mengeite has no commercial or industrial applications. It is solely an object of scientific interest and a collector's item for specialized collectors of rare minerals (so-called micromounts).

Properties

Luster
Vitreous to sub-metallic
Streak
Brown
Cleavage
Perfect on {010}
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of mengeite is possible almost exclusively through advanced analytical methods, such as energy-dispersive X-ray spectroscopy (EDS/EDX) to confirm chemical composition (presence of barium, manganese, magnesium, and phosphorus) and X-ray diffraction (XRD) to determine the crystal structure. Visually, under magnification, characteristic features include very small, dark brown to black, tabular crystals forming radial aggregates. ## Distinguishing from Similar Minerals It can be confused with other rare, dark manganese phosphates occurring in the same environment, such as minerals from the apatite group or especially other minerals from the Penrice quarry. Definitive differentiation requires specialized laboratory equipment. ## Crystal Forms Crystals are very small, tabular or bladed, often flattened. They usually occur as radial or rosette-like aggregates on the surface of the host rock.

Geological environment

## Genesis This mineral forms in phosphate deposits embedded in dolomitic rocks that have undergone contact metamorphism. Its crystallization is associated with hydrothermal or metasomatic activity that introduced or mobilized barium and manganese. It occurs in small fissures and vugs within the rock. ## Mineral Associations In its type locality (Penrice quarry), mengeite co-occurs with minerals such as apatite, barite, hematite, gatehouseite, shigaite, triploidite, and minerals from the pyroxene group. ## Localities The only confirmed and described occurrence of mengeite in the world is the Penrice quarry near Angaston, in the Barossa Valley region of South Australia. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a mengeite specimen is primarily assessed based on the abundance and development of its microscopic crystals. The most desirable specimens are those with clearly formed, radial aggregates that are well visible under a microscope. Contrast with a lighter host rock also enhances visual appeal. Due to its extreme rarity, any analytically confirmed specimen is valuable. ## Popular Localities The only source of specimens is the Penrice quarry in South Australia. Material from this locality is extremely difficult to acquire and appears on the collector's market sporadically.

Care and storage

## Cleaning Specimens containing mengeite, typically micromounts, should not be cleaned mechanically or chemically. The safest method for dust removal is using compressed air from a safe distance or a very soft brush (e.g., camel hair). ## What to Avoid Avoid contact with all chemicals, including acids and bases. The mineral is potentially sensitive to changes in temperature and humidity, so extreme conditions should be avoided. It should not be washed in water, especially with ultrasonic cleaning. ## Storage Specimens should be stored in stable conditions, away from dust, sunlight, and heat sources. The best way to store them is in dedicated micromount boxes, which protect delicate crystals from mechanical damage and contamination.

Sources

Read more