Cuatrocapaite-(K)

Chemical formula: K₃(NaMg&#₉₇₄₄;)(As₃+₂O₃)₆Cl₆·₁₆H₂O

Cuatrocapaite-(K) is an extremely rare arsenite halide forming unique, hexagonal crystals of an intense yellow color.

## Characteristics Cuatrocapaite-(K) is a very complex, hydrated arsenite of potassium, sodium, magnesium, and chlorine. It occurs as extremely small, but well-formed hexagonal crystals with a tabular habit. The crystals are transparent, have an intense, canary-yellow color, and exhibit a strong, vitreous luster. They typically form small, isolated flakes or small aggregates on the surface of the host rock. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2. It is brittle, and its density, calculated from the formula and cell parameters, is 2.83 g/cm³. The crystals exhibit perfect cleavage in one direction, parallel to the crystal base {0001}. ## Colors and Varieties The characteristic and only observed color of cuatrocapaite-(K) is intense canary-yellow. This mineral does not form color varieties. ## History and Name Cuatrocapaite-(K) was discovered in the Torrecillas mine in the municipality of Salar de Gorbea, Iquique Province, Chile. It was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 2019 (IMA2019-021). The name refers to its crystal structure, which consists of four layers ("cuatro capas" in Spanish), and to the potassium (K) dominant in its composition. ## Uses Due to its extreme rarity and microscopic crystal sizes, cuatrocapaite-(K) has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare minerals.

Properties

Mohs hardness: 2
Luster: Vitreous
Streak: Light yellow
Density: 2.83
Cleavage: Perfect on {0001}
Fracture: Brittle
Transparency: Transparent
Crystal system: Trigonal

Diagnostic features

## Identification Characteristic features of cuatrocapaite-(K) include its intense canary-yellow color, hexagonal, tabular crystal habit, and vitreous luster. Its occurrence as tiny, isolated crystals on the host rock in paragenesis with other rare arsenites is also a key indicator. However, definitive identification requires advanced analytical methods, such as Raman spectroscopy or X-ray diffraction. ## Distinguishing from Similar Minerals This mineral can be confused with other secondary, yellow arsenic minerals, such as pharmacolite or some arsenates. However, cuatrocapaite-(K) is distinguished by its unique hexagonal crystal habit and specific, intense shade of yellow. Co-occurrence with torrecillasite and other minerals from the type locality is a strong diagnostic clue. ## Crystal Forms Cuatrocapaite-(K) crystallizes in the form of thin, hexagonal tablets, often with sharply defined edges. Crystals are usually very small, not exceeding fractions of a millimeter, and occur singly or in small groups on the rock surface.

Geological environment

## Genesis Cuatrocapaite-(K) is a secondary mineral that forms in the oxidation zone (gossan) of polymetallic deposits rich in arsenic. It forms in arid conditions, as a result of the weathering processes of primary ore minerals, mainly arsenopyrite, in the presence of solutions rich in potassium, sodium, magnesium, and halogens. ## Mineral Associations This mineral occurs in direct association with other rare arsenites and sulfates. It is most commonly accompanied by torrecillasite, magnesiofluckite, lavendulan, annabergite, erythrite, gypsum, and quartz. ## Localities The only confirmed locality of cuatrocapaite-(K) in the world is its type locality – the Torrecillas mine, located near Salar de Gorbea in the Tarapacá region of northern Chile.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of cuatrocapaite-(K) specimens is primarily assessed based on crystal development and size, although they are always microscopic. The most desirable specimens are those with sharp, well-defined, isolated crystals of intense, saturated color, aesthetically set against the rock matrix. Aesthetic composition and the presence of associated minerals, especially the equally rare torrecillasite, are also important. ## Popular Localities The only source of specimens of this mineral is the Torrecillas mine in Chile. Material from this locality is extremely difficult to obtain and appears on the collector's market sporadically, usually offered by specialized dealers of rare minerals.

Care and storage

## Cleaning Specimens should be handled with utmost care. Cleaning is highly inadvisable due to the extreme brittleness and small size of the crystals. If absolutely necessary, a gentle stream of compressed air from a safe distance can be used to remove loose dust particles. ## What to Avoid Avoid contact with water and any chemicals, as the mineral is hydrated and can be damaged. The crystals are very soft and brittle, so any physical contact, including touching and mechanical cleaning, should be avoided. Protect the specimen from vibrations and shocks. ## Storage Cuatrocapaite-(K) specimens should be stored exclusively in stable, sealed "micromount" containers that protect them from mechanical damage, dust, and humidity changes. Avoid exposure to direct sunlight and extreme temperatures.

Sources

IMA Mineral List (CNMNC)