Hydrobasaluminite

Chemical formula: Al₄(S₆+O₄)(OH)₁₀·₁₅H₂O

A hydrated basic aluminum sulfate, occurring as white, brittle, and earthy masses or coatings.

## Characteristics Hydrobasaluminite is a hydrated basic aluminum sulfate, most commonly forming white or creamy-white, massive aggregates with an earthy or porcelaneous texture. Rarely, it appears as fine coatings or small reniform aggregates. Due to its microcrystalline nature, it does not form macroscopic, well-developed crystals. It is a very soft and brittle mineral, and its aggregates easily crumble into powder when rubbed between the fingers. ## Physical Properties This mineral is characterized by very low hardness, approximately 1 on the Mohs scale, meaning it can be scratched with a fingernail. Its luster is dull or earthy, and the mineral itself is opaque. The density is low, around 1.93 g/cm³. ## Colors and Varieties Hydrobasaluminite occurs almost exclusively in white, sometimes with a creamy or yellowish tint caused by impurities. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1948 by Gordon H. Bannister and George F. Claringbull. Its name refers to its chemical composition – it is a more hydrated ("hydro") form of another mineral, basaluminite. The type locality (locus typicus) is Lodge Gill in North Yorkshire, England. ## Applications Hydrobasaluminite has no industrial significance. It is solely an object of scientific interest and is sought after by collectors specializing in rare secondary minerals or minerals from specific localities.

Properties

Mohs hardness: 1
Luster: Dull
Streak: White
Density: 1.93
Cleavage: None
Fracture: Earthy
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Characteristic features of hydrobasaluminite include its white color, earthy or dull appearance, very low hardness (easily abraded), and occurrence as masses, crusts, and coatings. Due to its porous structure, it may tend to stick to the tongue. It usually occurs in paragenesis with other secondary minerals. ## Distinguishing from Similar Minerals It is visually impossible to distinguish from basaluminite – this requires X-ray diffraction (XRD) analysis to determine the degree of hydration. It can be confused with other white, earthy minerals, such as kaolinite, allophane, or gypsum. It is distinguished from gypsum by its significantly lower hardness. Kaolinite is often more "greasy" to the touch. ## Crystal Forms Hydrobasaluminite crystals are microscopic laths and are only visible under high magnification. Macroscopically, this mineral forms only compact, massive, earthy aggregates, less commonly reniform or crustal.

Geological environment

## Genesis Hydrobasaluminite is a secondary mineral, formed in the weathering zone of aluminum-rich rocks (e.g., clay shales, clays) under the influence of sulfate-rich waters. These sulfates often originate from the oxidation of sulfide minerals, mainly pyrite. It is a low-temperature mineral, typical of near-surface environments. ## Mineral Associations It most commonly co-occurs with minerals of similar genesis, such as basaluminite, allophane, gibbsite, as well as gypsum, quartz, and various clay minerals. ## Localities Key occurrences include: Lodge Gill in North Yorkshire (England, type locality), coal mines near Tatabánya (Hungary), and some localities in the USA, including Pennsylvania and Utah.

Rarity

Not very common

For collectors

## Quality Criteria Hydrobasaluminite is not a mineral valued for aesthetic qualities in the traditional sense. Its collector's value lies primarily in its rarity, origin from a known or classic locality, and scientific value. The most highly prized specimens are pure, of the largest possible mass, originating from the type locality or well-documented occurrences, often in association with other rare secondary minerals. ## Popular Localities Specimens from the type locality in Lodge Gill, England, are most sought after by specialized collectors. Materials from Hungary and the United States, which were used for research on this mineral, are also scientifically valuable.

Care and storage

## Cleaning Hydrobasaluminite specimens are extremely sensitive to water and mechanical damage. To remove dust, use only a very soft, dry brush or a gentle puff of compressed air from a safe distance. Any contact with water is unacceptable. ## What to Avoid Absolutely avoid contact with water, which can cause partial dissolution or structural changes to the mineral. It is sensitive to acids and other chemicals. As a hydrated mineral, it can undergo changes under high temperatures. Due to its low hardness, it is susceptible to scratching and abrasion. ## Storage It is recommended to store specimens in a dry place, preferably in a closed, airtight container (e.g., a "micromount" box), to protect them from dust, moisture, and mechanical damage. Avoid exposure to direct sunlight and sudden temperature changes.

Sources

IMA Mineral List (CNMNC)