Woodallite

Chemical formula: Mg<sub>6</sub>Cr<sup>3+</sup><sub>2</sub>(OH)<sub>16</sub>Cl<sub>2</sub>·4H<sub>2</sub>O

Woodallite is a rare, purple-red mineral from the hydrotalcite group, occurring as microscopic platy crystals.

## Characteristics Woodallite is a hydrated magnesium chromium hydroxide chloride, belonging to the hydrotalcite group. It forms very small, hexagonal or pseudohexagonal platy crystals, typically less than 0.1 mm in diameter. It most often occurs as disseminated aggregates, coatings, or as a component of very fine-grained mineral mixtures. Due to the size of the crystals, its visual features are difficult to observe without a microscope. ## Physical Properties This mineral is characterized by very low hardness, estimated at approximately 1.5-2 on the Mohs scale. It has perfect cleavage in one plane, which is typical for minerals with a layered crystal structure. Its luster is described as waxy to vitreous. ## Colors and Varieties Woodallite exhibits colors ranging from purple, through reddish-purple, to purplish-red. No varieties have been distinguished. ## History and Name The mineral's name honors Roy Woodall (born 1930), an Australian geologist who made significant contributions to the development of mining and exploration in Australia. Woodallite was first described in 2001 based on material from the Mount Keith mine in Western Australia.

Properties

Mohs hardness
1.5 - 2
Luster
Waxy to Vitreous
Streak
Pale purple
Density
2.13
Cleavage
Perfect on {0001}
Fracture
Micaceous
Transparency
Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of woodallite is possible almost exclusively using advanced analytical methods, such as X-ray diffraction (XRD) or energy-dispersive X-ray spectroscopy (EDS), due to its microscopic size. Its characteristic purplish-red color and co-occurrence with other chromium minerals in ultramafic rocks can serve as a preliminary indication. ## Distinguishing from Similar Minerals It can be confused with other purple secondary minerals, such as stichtite or iowaite, which also belong to the hydrotalcite group. Stichtite, however, is usually lighter, with a lilac color, while iowaite is bluish-green to white. Definitive differentiation requires chemical analysis confirming the dominance of chromium and magnesium. ## Crystal Forms Woodallite forms small, hexagonal, platy crystals, often aggregated into rosette-like or spherical clusters. Crystals rarely exceed 50 micrometers.

Geological environment

## Genesis Woodallite is a secondary mineral, formed as a result of low-temperature hydrothermal processes or weathering of serpentinized, chromium-rich ultramafic rocks. It forms in veins and fractures cutting through these rocks. ## Mineral Associations It most commonly co-occurs with chromite, iowaite, serpentine, chlorite, talc, magnesite, brucite, and stichtite. ## Localities The most important and type localities for this mineral are the Mount Keith nickel mine and the Westonia gold mine, both in Western Australia. It has also been identified in the Bushveld Complex in the Republic of South Africa and in Norway.

Rarity

Very rare

For collectors

## Quality Criteria As a microscopic mineral, woodallite is valued primarily by collectors specializing in mineral systematics or "micromount" specimens. The value of a specimen is determined by the abundance and distinctness of microcrystals on the rock matrix, as well as the intensity of their purplish-red color. Association with other rare minerals is also important.

Care and storage

## Cleaning Due to its extreme softness and small crystal size, mechanical cleaning is inadvisable and practically impossible without damaging the specimen. If absolutely necessary, compressed air can be used from a safe distance to remove dust. ## What to Avoid Avoid contact with water and chemicals, which can dissolve or damage the delicate crystals. The mineral is very soft, so it should be protected from any physical contact and abrasion. Ultrasonic cleaning devices should also be avoided. ## Storage Woodallite specimens should be stored exclusively in closed, stable containers, such as "micromount" boxes, to protect them from mechanical damage, dust, and humidity changes.

Sources

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