Scarbroite

Chemical formula: Al<sub>5</sub>(CO<sub>3</sub>)(OH)<sub>13</sub>·5H<sub>2</sub>O

Scarbroite is a rare, hydrated aluminum carbonate, forming white, soft, and porous aggregates with an earthy appearance.

## Characteristics Scarbroite is a rare, hydrated aluminum carbonate. It occurs as earthy, chalk-like, or porcelain-like masses, often filling fissures and cracks in rocks. Its aggregates are typically white, very light, and porous, with a dull or earthy luster. Due to its softness and fragility, it is a mineral difficult to obtain in the form of well-developed specimens. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. It is exceptionally light, with a density close to that of water, which is its characteristic feature. It has a dull or earthy luster and is opaque. It exhibits perfect cleavage in one direction, but this is rarely observable due to its massive mode of occurrence. ## Colors and Varieties Scarbroite is usually white, but can also be colorless or slightly tinted gray, cream, or yellowish due to impurities. No distinct color varieties or commercial forms are recognized. ## History and Name The mineral's name comes from its first discovery location – Scarborough in North Yorkshire, England. It was first described in 1829 by William Vernon. For many years, its status as a distinct mineral species was questioned, but it was finally confirmed by the International Mineralogical Association (IMA) in 1979. ## Uses Scarbroite has no industrial applications. It is solely an object of interest for collectors of rare minerals and scientists.

Properties

Mohs hardness
2
Luster
Earthy
Streak
White
Density
1.9-2.1
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Scarbroite can be identified by its characteristic appearance – white, light, chalk-like, or porcelain-like masses. It is very soft (scratches with a fingernail) and exceptionally light. It often adheres to the tongue due to its porosity. It reacts with acids, releasing carbon dioxide bubbles. ## Distinguishing from Similar Minerals It can be confused with kaolinite, gibbsite, or other clay minerals. It is distinguished from kaolinite by its reaction with acid. Gibbsite is harder and denser. Other similar carbonate minerals, such as hydrozincite, often exhibit fluorescence under UV light, which scarbroite does not possess. ## Crystal Forms Scarbroite very rarely forms microscopic, platy crystals. It typically occurs as compact, earthy, chalk-like, or porcelain-like aggregates, as well as veins and fissure fillings.

Geological environment

## Genesis It is a secondary mineral, forming in the weathering zone of aluminum-rich rocks, such as sandstones, under low-temperature conditions. It forms as a result of carbonate-containing solutions acting on aluminum minerals. ## Mineral Associations It often co-occurs with quartz, calcite, gibbsite, as well as other clay minerals and carbonates. ## Localities The most known locality, which is also the type locality, is Scarborough in England. This mineral has also been found in Greece (mines in the Attica region), Hungary (Gánt), Russia (Southern Urals), and several locations in the USA (e.g., Nevada).

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those of the highest purity and whiteness, exhibiting a porcelain-like rather than merely earthy texture. Due to its rarity, any well-documented specimen from a confirmed locality is valuable. Veins of scarbroite in a contrasting rock matrix are particularly sought after. ## Popular Localities Classic and most desirable specimens come from the historical locality in Scarborough, England. Material from Greek mines in Lavrion is also prized in the collector's market.

Care and storage

## Cleaning Scarbroite specimens are extremely delicate, soft, and porous. They should not be cleaned wet, as they can absorb water and disintegrate. Dust can be removed very carefully using a soft brush or a stream of compressed air from a distance. ## What to Avoid Avoid contact with water and all chemicals, including acids, which can dissolve it. The mineral is very soft and brittle, so it should be protected from impacts, scratches, and vibrations. It should not be heated, as it contains water in its structure. ## Storage Scarbroite specimens are best stored in tightly sealed display boxes, away from dust, moisture, and direct sunlight. Each specimen should be isolated to prevent abrasion and mechanical damage.

Sources

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