Mengxianminite

Chemical formula: Ca₂Sn₄+₂Mg₃Al₈[(BO₃)(BeO₄)O₆]₂

Mengxianminite is an extremely rare mineral from the nigerite group, a transparent, colorless calcium, tin, magnesium, and aluminum borate-beryllium-oxide.

## Characteristics Mengxianminite is an extremely rare mineral, a member of the nigerite supergroup. It occurs as small, tabular crystals with a hexagonal outline, reaching sizes up to 0.3 x 0.3 x 0.1 mm. It is colorless and transparent, with a vitreous luster. Its unique chemical composition, including beryllium, boron, tin, magnesium, and aluminum, makes it an exceptional subject for scientific research. ## Physical properties Mengxianminite crystals exhibit a hardness of approximately 8 on the Mohs scale. The mineral is brittle, and its density, calculated based on composition and unit cell parameters, is 4.33 g/cm³. It does not exhibit fluorescence under ultraviolet light. ## Colors and varieties The mineral is colorless. No colored or commercial varieties are known. ## History and name The mineral's name honors Meng Xianmin (born 1934), a Chinese geologist and discoverer of the Xianghualing deposit, where this mineral was first identified. Mengxianminite was approved as a new mineral species by the International Mineralogical Association (IMA) in 2016 (IMA2016-063). It was described by a team of scientists led by Guowu Li. ## Uses Due to its extreme rarity and microscopic crystal sizes, mengxianminite has no industrial applications. It is solely of scientific significance and is an object of interest for specialized collectors of rare minerals.

Properties

Mohs hardness: 8
Luster: Vitreous
Streak: White
Density: 4.33
Cleavage: None
Fracture: Uneven
Transparency: Transparent
Crystal system: Trigonal

Diagnostic features

## Identification Identification of mengxianminite is possible only through advanced laboratory methods, such as Raman spectroscopy, X-ray microanalysis (EDS/WDS), and single-crystal X-ray diffraction. Visually, at the occurrence site, it can be suspected based on its co-occurrence with other minerals from the Xianghualing deposit, by observing colorless, hexagonal crystals under high magnification. ## Differentiation from similar minerals It can be confused with other colorless, hexagonal minerals from the nigerite group, such as nigerite or ferronigerite, as well as with phenakite or chrysoberyl, which also occur in the same environment. Definitive differentiation requires specialized chemical and structural analysis to reveal the unique presence and proportions of calcium, tin, magnesium, beryllium, and boron. ## Crystal forms Mengxianminite forms thin, tabular crystals with a hexagonal outline. Crystals are usually very small, not exceeding 0.3 mm in diameter.

Geological environment

## Genesis Mengxianminite forms in altered (greisenized) granites. It is a hydrothermal mineral, crystallizing in the late stages of magmatic evolution, in an environment rich in elements such as tin, beryllium, boron, and lithium. ## Mineral associations This mineral occurs in association with other minerals typical of the Xianghualing deposit, such as phlogopite, fluorite, liberite, phenakite, chrysoberyl, calcite, cassiterite, nigerite, taaffeite, and hsianghualite. It is found as inclusions in calcite or on phlogopite. ## Localities The only confirmed locality (type locality) of mengxianminite in the world is the Xianghualing tin deposit, located in Linwu County, Chenzhou Prefecture, Hunan Province, China.

Rarity

Extremely rare

For collectors

## Quality criteria As an extremely rare micromineral, the main criterion for value is the confirmed presence of mengxianminite on the specimen itself. The most highly prized specimens are those with well-formed, sharp crystals that are as large as possible (though still microscopic) and isolated on the rock matrix. Contrast with associated minerals, e.g., dark phlogopite, enhances aesthetic appeal. ## Popular localities The only source of specimens is the type locality – the Xianghualing deposit in China. Specimens from this location are highly sought after by collectors specializing in systematics and rare minerals (so-called "rare species collectors").

Care and storage

## Cleaning Due to their microscopic size and embedding in the host rock, specimens do not require and should not undergo cleaning. Any attempt at mechanical or chemical cleaning risks irreversible destruction of the crystals. ## What to avoid Avoid contact with any chemicals, ultrasound, and sudden temperature changes. Specimens should be protected from dust and mechanical damage. ## Storage Mengxianminite specimens, as microminerals, should be stored in specialized "micromount" boxes that protect them from physical damage and dust. Exposure to vibrations and shocks should be avoided.

Sources

IMA Mineral List (CNMNC)