Alumohydrocalcite

Chemical formula: CaAl<sub>2</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>4</sub>·3H<sub>2</sub>O

A hydrous basic calcium aluminum carbonate, most commonly forming white, chalk-like coatings and spherical aggregates.

## Characteristics Alumohydrocalcite is a mineral from the carbonate group, characterized by its softness and most often an earthy or chalk-like appearance. It usually occurs as white, porous masses, crusts, and coatings. More visually appealing forms include small, spherical aggregates (spherulites) with a radial, fibrous internal structure or botryoidal aggregates. Well-formed, individual crystals are extremely rare and appear as microscopic, acicular or bladed forms. ## Physical Properties It is a very soft mineral, with a Mohs hardness of about 2.5, meaning it can be scratched with a fingernail. Its density is low, ranging from 2.23-2.41 g/cm³. The luster on fracture surfaces can be pearly or silky, but it is most often dull or earthy. The mineral is translucent in thin fragments and opaque in thicker masses. ## Colors and Varieties Alumohydrocalcite is usually chalk-white. Less commonly, it can be colorless, grayish, or yellowish, which is a result of impurities. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1926 by W. N. Chirvinsky. Its name, given by the discoverer himself, directly refers to its chemical composition: "alumo" from aluminum, "hydro" from the presence of water and hydroxyl groups, and "calcite" from calcium and the carbonate group.

Properties

Mohs hardness
2.5
Luster
Pearly
Streak
White
Density
2.23 - 2.41
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Translucent to Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Alumohydrocalcite can be identified by its characteristic low hardness (can be scratched with a fingernail), white color, and often chalk-like or earthy form. A key diagnostic feature is its vigorous reaction with dilute hydrochloric acid, indicating the presence of carbonates. However, confirmation requires chemical analysis to detect aluminum. ## Distinguishing from Similar Minerals It can be confused with other white, earthy carbonates such as calcite, aragonite, or huntite. It differs from calcite and aragonite by its significantly lower hardness. Distinguishing it from huntite, which is also a soft carbonate, is practically impossible in the field and requires advanced methods such as X-ray diffraction (XRD). It can also be confused with clay minerals (kaolinite) or gibbsite, which, however, do not react with acid. ## Crystal Forms Crystals are microscopic, acicular, or fibrous. They form radial aggregates that build spherical spherulites, botryoidal aggregates, as well as coatings, efflorescences, and earthy or chalky masses.

Geological environment

## Genesis Alumohydrocalcite is a secondary mineral, forming under low-temperature conditions. It forms as a result of the weathering of rocks rich in aluminum and calcium, such as limestones, marls, or serpentinites. It is also a product of low-temperature hydrothermal activity. It is often found in the oxidation zones of ore deposits, in caves as a component of some speleothems, and as a precipitate in mine waters. ## Mineral Associations It most commonly co-occurs with other secondary minerals such as calcite, aragonite, hydromagnesite, gibbsite, allophane, and also with minerals from the serpentine group. ## Localities Significant occurrences of alumohydrocalcite are found worldwide, although it rarely forms large accumulations. Classic localities include the area around Lake Saki in Crimea (Ukraine), where it was discovered. Beautiful spherulitic specimens come from Dubník in Slovakia, the mine in Tata in Hungary, and Tarna Mare in Romania. It also occurs in Italy (Vesuvius), Austria, Germany, and the United States.

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens in the form of well-formed, undamaged spherulites or aesthetic botryoidal aggregates. The purity and intensity of the white color are important. Specimens with a contrasting matrix or in association with other colored minerals (e.g., in geodes) are more highly valued. Earthy and chalk-like masses have little collector's value. ## Popular Localities The most classic and desired specimens are spherulitic ones from European localities, especially from Hungary (e.g., Tata, Iszkaszentgyörgy) and Slovakia (Dubník). These localities have yielded specimens of exceptionally good formation and aesthetics.

Care and storage

## Cleaning Alumohydrocalcite specimens are extremely delicate and sensitive. They should only be cleaned mechanically, dry, using a very soft brush to remove dust. Absolutely avoid water and any liquids. ## What to Avoid The mineral is very soft and brittle, susceptible to mechanical damage. It reacts violently and dissolves in acids. Contact with water should be avoided, as it can damage its structure, especially in the case of porous aggregates. Store away from moisture and heat sources. ## Storage It is recommended to store specimens in separate, padded boxes to prevent abrasion and damage. It should not be placed in the same compartment with harder minerals. Display should be in a closed cabinet, protecting against dust and humidity changes.

External references

Sources

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