Caichengyunite

Chemical formula: Fe²⁺₃Al₂(SO₄)₆·₃₀H₂O

Caichengyunite is a very rare, hydrated iron and aluminum sulfate, forming small, platy, grayish-white crystals.

## Characteristics Caichengyunite is a mineral from the sulfate group, chemically classified as a hydrated iron and aluminum sulfate. It occurs as very small, platy crystals, rarely exceeding 0.5 mm in length. Its typical appearance is colorless to grayish-white, transparent flakes with a vitreous luster. Due to its extreme rarity and small crystal size, it is a mineral known primarily within the scientific community. ## 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 also very light, with a density of approximately 2.20 g/cm³. The crystals are transparent and exhibit a vitreous luster. ## Colors and Varieties Caichengyunite is typically colorless or has a grayish-white hue. No colored or commercial varieties have been distinguished. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2023 under number IMA2023-030. Its name honors Professor Chengyun Cai (born 1937), a Chinese geologist specializing in ore deposit geology, for his contributions to geological research in China. It was discovered in the Longshu lead-zinc deposit in Sichuan Province, China. ## Uses Due to its extreme rarity, small crystal size, and poor durability, caichengyunite has no industrial applications. Its significance is purely scientific and collectible, although it is practically unavailable on the market.

Properties

Mohs hardness: 1.5-2
Luster: Vitreous
Streak: White
Density: 2.20
Transparency: Transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Amateur identification of caichengyunite is practically impossible. It requires advanced analytical techniques such as X-ray diffraction (XRD) and chemical analysis (EDS). Its main characteristics include very low hardness (1.5-2), occurrence as small, platy crystals, and genesis in the oxidation zones of sulfide deposits. ## Distinguishing from Similar Minerals It can be confused with other secondary sulfates, such as gypsum or melanterite, which are also soft and often colorless. Differentiation requires specialized equipment. Unlike gypsum, caichengyunite is associated with weathering zones of metal deposits, not evaporites. ## Crystal Forms It forms very small, thin, platy crystals with a hexagonal outline, reaching up to 0.5 mm. It occurs as individual crystals or small aggregates on the surface of other minerals.

Geological environment

## Genesis Caichengyunite is a secondary mineral, formed in the oxidation zone (gossan) of polymetallic (Pb-Zn) deposits. It forms as a result of the weathering of primary sulfide minerals, such as pyrite, in the presence of clay minerals, which provide aluminum. ## Mineral Associations At its type locality, it is associated with anglesite, gypsum, goethite, quartz, and clay minerals. ## Localities The only confirmed occurrence of caichengyunite in the world is its type locality: the Longshu lead-zinc deposit, Huidong County, Liangshan Yi Autonomous Prefecture, Sichuan Province, China.

Rarity

Extremely rare

For collectors

## Quality Criteria As a "micromount" type mineral, the quality of a specimen is primarily determined by the presence of well-formed, sharp, and undamaged crystals, even if they are microscopic in size. Specimens where crystals are clearly visible on a contrasting substrate (matrix) are desirable. Due to its extreme rarity, any analytically confirmed specimen is valuable for specialized collections. ## Popular Localities The only source of specimens is the type locality in China. This mineral is practically unavailable on the open collector's market and primarily goes to research institutions and museums.

Care and storage

## Cleaning Caichengyunite specimens are extremely delicate and sensitive to water. They should not be wet cleaned. The only safe method for dust removal is to use a soft brush or carefully blow air from a safe distance. ## What to Avoid Contact with water, chemicals, and high temperatures, which can cause dehydration and destruction of the mineral, must be strictly avoided. As a very soft mineral, it is susceptible to any scratches and mechanical damage. ## Storage Specimens should be stored in stable, dry conditions, preferably in a sealed "micromount" container, which protects against dust, moisture, and physical damage. Avoid exposure to direct sunlight and temperature changes.

External references

https://www.mindat.org/min-39649.html