Calclacite

Chemical formula: Ca(CH₃COO)₁-Cl·₅H₂O

Calclacite is a hydrated calcium chloride-acetate, a secondary mineral forming as efflorescences on limestone rocks stored in oak display cases.

## Characteristics Calclacite is a mineral from the group of organic compounds that forms almost exclusively under artificial conditions, as a result of chemical reactions occurring in museum collections. Typical specimens appear as white, silky efflorescences and crusts on the surface of calcium carbonate-rich rocks and fossils, such as limestones, marbles, or shells. It consists of very fine, needle-like crystals arranged in tangled aggregates, which are extremely delicate and fragile to the touch. ## Physical Properties This mineral is very soft and brittle. It is characterized by a silky luster and a low density of approximately 1.51 g/cm³. It is translucent and readily dissolves in water, which is one of its key diagnostic features. ## Colors and Varieties Calclacite occurs exclusively in white. No colored varieties or trade names are known. ## History and Name The mineral's name, given in 1947 by René van Tassel, comes from its chemical composition: **calc**ium, **chl**oride, and **ac**etate. It was first identified on specimens from the collection of the Royal Belgian Institute of Natural Sciences in Brussels, where it formed as a degradation product of exhibits stored in oak cabinets. ## Uses Calclacite has no industrial applications. It is mainly of scientific interest as an example of a mineral forming in an anthropogenic environment and is considered a conservation problem in museums.

Properties

Luster: Silky
Streak: White
Density: 1.51
Cleavage: Perfect on {100}
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification The key diagnostic feature of calclacite is the context of its occurrence – white, acicular efflorescences on limestone rocks or fossils from old museum collections, especially those stored in oak furniture. The mineral is extremely soft, has a silky luster, and completely dissolves in water. The reaction with water is a destructive test but ultimately confirms its identity. ## Distinguishing from Similar Minerals Calclacite can be confused with other white, fibrous secondary minerals (efflorescences), such as gypsum, aragonite, or other evaporite minerals. However, it is distinguished by its specific formation environment (reaction with acetic acid vapors from wood) and its complete and rapid solubility in water, which is not typical for most carbonates or sulfates of similar appearance. ## Crystal Forms It forms very fine, elongated, acicular crystals. These crystals almost always occur as tangled fibrous aggregates, forming efflorescences, crusts, and coatings on the surface of the host rock.

Geological environment

## Genesis Calclacite is a mineral of anthropogenic genesis. It forms in a specific microenvironment, namely enclosed museum cabinets and display cases made of wood, especially oak. This wood emits volatile organic compounds, including acetic acid. Acetic acid vapors react with calcium carbonate (calcite or aragonite) forming geological or paleontological specimens. The presence of chloride ions is also necessary for the reaction, which can come from the specimen itself, groundwater, contamination, or even human hands. ## Mineral Associations This mineral does not form typical associations with other minerals in a geological sense. It occurs directly on a substrate rich in calcium carbonate, such as calcite, aragonite (e.g., in shells), or limestone and marble rocks. ## Localities Calclacite localities are not geological sites but museum collections worldwide. It has been identified in many institutions where old collections were stored in wooden display cases. The type locality is the Royal Belgian Institute of Natural Sciences in Brussels (Belgium).

Rarity

Rare

For collectors

## Quality Criteria As a mineralogical curiosity, calclacite is valued not for the beauty of individual crystals, but for the abundance and delicacy of fibrous aggregates covering an interesting host specimen (e.g., a well-preserved fossil). The value of a specimen is enhanced by a well-documented history of its origin and formation. Due to its nature, it is a mineral for specialized collectors of rare and unusual minerals. ## Popular Localities The most famous "localities" are historical mineralogical and paleontological collections in Europe and North America. Specimens do not come from mines or geological outcrops, and their only source is museum collections.

Care and storage

## Cleaning The mineral is water-soluble, so **it must not be cleaned wet**. Any attempts to clean it with water or other liquids will lead to its permanent destruction. Only very careful dust removal with a soft, dry brush or a gentle stream of compressed air from a safe distance is permissible. ## What to Avoid Contact with water and other liquids must be strictly avoided. Calclacite is hygroscopic, meaning it absorbs moisture from the air, so it must be protected from moisture and sudden changes in humidity. It is also very sensitive to mechanical damage. It should not be touched, as it is extremely brittle. ## Storage Calclacite specimens require storage under stable conditions. It is best to place them in a sealed, airtight container (e.g., a membrane box or crystallizer) with a desiccant, such as silica gel. Avoid storing them in wooden boxes, especially oak, to prevent further chemical reactions.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FCalclacite