Lanmuchangite

Chemical formula: Tl₁+Al(S₆+O₄)₂·₁₂H₂O

Lanmuchangite is a very rare, colorless thallium alum, occurring as small, octahedral crystals.

## Characteristics Lanmuchangite is an extremely rare mineral from the alum group, being a hydrated thallium and aluminum sulfate. It occurs as very small, well-formed, colorless crystals in the shape of regular octahedra, reaching up to 0.5 mm in size. It is isostructural with potassium alum. Due to its chemical composition, containing toxic thallium, it is a mineral of scientific and collecting interest only. ## Physical Properties Lanmuchangite crystals exhibit a vitreous luster. This mineral is soft and brittle. Its density is higher than typical alums due to the presence of heavy thallium. ## Colors and Varieties Lanmuchangite is colorless. No colored varieties are known. ## History and Name The mineral's name comes from its discovery locality – the Lanmuchang mercury-thallium deposit in Guizhou Province, China. It was approved as a new mineral by the International Mineralogical Association (IMA) in 1994. ## Applications Due to its extreme rarity, small crystal sizes, and the toxicity of thallium, lanmuchangite has no practical applications and is solely an object of scientific interest and for advanced collectors of rare minerals.

Properties

Mohs hardness: 2-3
Luster: Vitreous
Streak: White
Density: 3.22
Cleavage: None
Fracture: Conchoidal
Transparency: Transparent
Crystal system: Cubic

Diagnostic features

## Identification Amateur identification of lanmuchangite is practically impossible due to its microscopic size and appearance typical of alums. Certain identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS) to confirm its chemical composition (presence of Tl, Al, and S). ## Differentiation from Similar Minerals It can be confused with other alums, such as potassium alum. Differentiation is only possible based on chemical analysis, which will show the presence of thallium instead of potassium. ## Crystal Forms It forms exclusively small, perfectly developed crystals with a regular octahedral habit.

Geological environment

## Genesis Lanmuchangite is a secondary mineral, formed in the oxidation zones of hydrothermal deposits rich in thallium, arsenic, and antimony. It forms as a result of the weathering of primary thallium-bearing minerals, such as lorandite or vrbaite, in the presence of sulfates derived from pyrite oxidation. ## Mineral Associations It most commonly co-occurs with other rare thallium minerals, such as lorandite, and with sulfates like gypsum and jarosite. It is also accompanied by realgar, orpiment, and pyrite. ## Localities The only confirmed occurrence of this mineral in the world is its type locality – the Lanmuchang deposit, in Xingren County, Guizhou Province, southwestern China.

Rarity

Extremely rare

For collectors

## Quality Criteria Due to its extreme rarity and microscopic size, every analytically confirmed specimen is valuable. Samples showing well-formed, even if tiny, octahedral lanmuchangite crystals, embedded on a contrasting rock matrix, are most highly prized. Association with other rare thallium minerals is also important.

Care and storage

## Cleaning Lanmuchangite specimens should not be wet cleaned. The mineral is water-soluble. A soft brush or carefully used photographic blower can be used to remove dust. ## What to Avoid Absolutely avoid contact with water and other liquids that can dissolve the mineral. Store away from moisture and heat sources that could cause dehydration. Due to its thallium content, avoid direct skin contact, inhaling dust, and wash hands after any contact with the mineral. ## Storage It is recommended to store in sealed, dry containers (e.g., "perky box" type) or in specialized display cases with humidity control. It should be isolated from other minerals that it could damage upon contact.

Sources

IMA Mineral List (CNMNC)