Hydrocalumite

Chemical formula: Ca<sub>4</sub>Al<sub>2</sub>(OH)<sub>12</sub>(Cl,CO<sub>3</sub>,OH)<sub>2</sub>·4H<sub>2</sub>O

Hydrocalumite is a rare, hydrated calcium-aluminum hydroxychloride-carbonate, forming soft, platy crystals with a pearly luster.

## Characteristics Hydrocalumite is a mineral from the hydrotalcite group, chemically classified as a hydrated calcium-aluminum hydroxychloride-carbonate. It forms very small, thin, tabular crystals with a hexagonal outline. It most commonly occurs as platy, scaly, or radiating aggregates resembling rosettes. Specimens are typically white or colorless, with a characteristic pearly luster on cleavage surfaces. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2.5, meaning it can be scratched with a fingernail. It is lightweight, with a density of approximately 2.2 g/cm³. It exhibits perfect cleavage in one direction, parallel to the crystal base, which causes it to easily separate into thin lamellae. The luster is pearly on cleavage planes and waxy on other surfaces. It is transparent to translucent. ## Colors and Varieties Hydrocalumite is usually colorless or white. No distinct color varieties or commercial names are recognized. ## History and Name The mineral's name refers to its chemical composition: "hydro" from the presence of water in the structure, "cal" from calcium (Latin *calx*), and "umite" from aluminum. It was first described in 1934 by Cecil Edgar Tilley based on specimens found at Scawt Hill in County Antrim, Northern Ireland. ## Applications Hydrocalumite has no industrial applications. However, it is an important phase product formed during the hydration of Portland cement, making it a subject of research in materials engineering. For collectors, it is an interesting and rare mineral, especially specimens from classic localities.

Properties

Mohs hardness
2.5
Luster
Pearly to Waxy
Streak
White
Density
2.21 - 2.26
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Characteristic features of hydrocalumite include its platy or scaly habit, white color, pearly luster on cleavage surfaces, and very low hardness. It reacts with hydrochloric acid. Its occurrence in paragenesis with other contact-metamorphic minerals or in concrete is also an important clue. ## Distinguishing from Similar Minerals Hydrocalumite is sometimes confused with other white, platy minerals. It can be distinguished from: - **Brucite**: has similar hardness but often a greasy or waxy luster and a different formation environment. - **Gibbsite**: is slightly harder (2.5-3.5 on the Mohs scale). - **Talc**: is much softer (hardness 1) and feels soapy or greasy to the touch. Final identification often requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Crystals are thin-tabular, with a hexagonal outline. It usually forms platy, scaly, foliated, or radiating rosette aggregates. It rarely occurs as single, well-formed crystals.

Geological environment

## Genesis Hydrocalumite is a secondary mineral, formed under low-temperature hydrothermal alteration conditions of calcium-rich rocks, such as skarns and metamorphosed limestones. It is also a common, though microscopic, component of hardened cement and concrete, where it crystallizes as a result of the hydration reactions of calcium aluminates. ## Mineral Associations In natural environments, it co-occurs with minerals such as ettringite, portlandite, afwillite, larnite, calcite, and aragonite. ## Localities The most important natural occurrences of hydrocalumite include Scawt Hill in Northern Ireland (type locality), Boisséjour in France, Ettringen and Hegau in Germany, Klöch in Austria, and Crestmore in California, USA. As an artificial product, it is present in concrete structures worldwide.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, radiating aggregates (rosettes) or visible, even microscopic, crystals with a hexagonal outline are most valued by collectors. Paragenesis with other rare minerals from known localities, especially from the type locality at Scawt Hill, is also highly appreciated. Due to the small size of the crystals, it is primarily a mineral for micromineral collectors. ## Popular Localities Specimens from Scawt Hill in Northern Ireland are considered classic and most desirable. Interesting finds can also come from the Crestmore mine in California and from German volcanic localities.

Care and storage

## Cleaning Hydrocalumite specimens are extremely delicate and soft. To remove dust, use only a soft brush or a stream of compressed air from a safe distance. Avoid contact with water, which can damage or alter the mineral's structure. ## What to Avoid The mineral is sensitive to all chemicals, especially acids, which rapidly dissolve it. It should be protected from moisture, high temperatures, and direct sunlight. Due to its perfect cleavage and low hardness, it is very susceptible to mechanical damage. ## Storage It is recommended to store specimens in closed, padded boxes or display cases to protect them from dust, light, and physical damage. It should not be placed near harder minerals that could scratch it.

Sources

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