Carrboydite

Chemical formula: Ni₂+₁₄Al₉(S₆+O₄)₆(OH)₄₃·₇H₂O

A very rare, secondary nickel and aluminum sulfate, forming characteristic pale blue-green coatings and aggregates.

## Characteristics Carrboydite is a hydrated nickel and aluminum sulfate, belonging to the group of rare secondary minerals. It most often occurs as microcrystalline, earthy, or powdery coatings and crusts. It also forms small spherical or botryoidal aggregates. Its appearance is usually inconspicuous, and specimens rarely exceed a few millimeters in size. ## Physical Properties This mineral is very soft, with a Mohs hardness of about 2. It is brittle and has a density of 2.86 g/cm³. The luster is most often waxy, dull, or earthy. It is a translucent mineral, becoming opaque in thicker aggregates. ## Colors and Varieties Carrboydite is characterized by a typical, pale, blue-green or turquoise color, associated with the presence of nickel ions in its structure. No color varieties or commercial varieties are distinguished. ## History and Name The mineral's name comes from its discovery locality – the Carr Boyd nickel mine, located north of Kalgoorlie in Western Australia. It was described and recognized as a new mineral species in 1976 by E. H. Nickel and R. M. Clarke. ## Applications Due to its extreme rarity and small size of occurrences, carrboydite has no industrial application. It is solely an object of scientific and collecting interest.

Properties

Mohs hardness: 2
Luster: Waxy to Dull
Streak: Pale greenish white
Density: 2.86
Cleavage: None
Fracture: Earthy
Transparency: Translucent to Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Carrboydite is identified by its characteristic pale blue-green color, low hardness (about 2 on the Mohs scale), and typical occurrence as earthy coatings and spherical aggregates. Its paragenesis – occurrence in the oxidation zones of nickel sulfide deposits – is also crucial. ## Distinguishing from Similar Minerals This mineral can be confused with other secondary nickel minerals of similar color, such as takovite, hydrohonessite, glaucocerinite, or gaspéite. Gaspéite, being a carbonate, will react with hydrochloric acid, whereas carrboydite (a sulfate) does not exhibit such a reaction. Final differentiation from other nickel sulfates and hydroxides often requires advanced analytical methods, such as EDS spectroscopy or X-ray diffraction (XRD). ## Crystal Forms Carrboydite does not form macroscopic, well-developed crystals. It occurs exclusively in the form of microcrystalline aggregates: earthy, powdery masses, as well as botryoidal or spherulitic crusts.

Geological environment

## Genesis Carrboydite is a secondary mineral, formed in the oxidation (weathering) zones of nickel and copper sulfide deposits. It forms as a result of the slow decomposition of primary ore minerals, such as pentlandite, violarite, millerite, or chalcopyrite, under conditions of low temperature and pressure. ## Mineral Associations This mineral often co-occurs with other secondary weathering products of sulfides. Its most common associated minerals include: takovite, hydrohonessite, glaucocerinite, gaspéite, pekoite, as well as gypsum, epsomite, morenosite, and goethite. ## Localities The most important and type locality is the Carr Boyd nickel mine in Western Australia. It is also known from several other localities worldwide, including the Lord Brassey mine in Tasmania (Australia), from ancient dumps in Laurion (Lavrion), Greece, and from Goms in the canton of Valais, Switzerland.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of carrboydite specimens depends primarily on the intensity and purity of its pale blue-green color. Specimens with distinctly formed botryoidal aggregates are more highly valued than those in the form of earthy coatings. The size of the coverage on the matrix rock and the presence of rare associated minerals, which increase the scientific and visual value of the specimen, are also important. ## Popular Localities The best-known specimens, although usually microscopic in size, come from the type locality in Australia (Carr Boyd Mine). Collector material is also sometimes found in the historical mines of Laurion, Greece, which are known for their abundance of rare secondary minerals.

Care and storage

## Cleaning Carrboydite specimens are very delicate and sensitive to water. To remove dust, use only a soft brush or compressed air at low pressure. Contact with water, especially hot water, can lead to partial dissolution or chemical alteration of the mineral. ## What to Avoid Contact with water, acids, and other chemicals should be strictly avoided. The mineral is very soft and susceptible to scratching. It should be protected from impacts and abrasion. It is also sensitive to elevated temperatures, which can cause dehydration (loss of water from its structure) and its destruction. ## Storage It is recommended to store specimens in a dry place, preferably in closed, airtight "perky boxes." This protects the mineral from dust, moisture, and mechanical damage.

Sources

IMA Mineral List (CNMNC)