Walthierite

Chemical formula: Ba<sub>0.5</sub>Al<sub>3</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>

Walthierite is a rare aluminum and barium sulfate, forming tiny, platy crystals with a pearly luster, occurring in hydrothermal zones.

## Characteristics Walthierite is a complex, hydrated aluminum and barium sulfate, belonging to the alunite group. It occurs as very small, hexagonal or pseudohexagonal, platy crystals, rarely exceeding 0.5 mm. These crystals often form rosette-like or lamellar aggregates. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties This mineral is characterized by a Mohs hardness of approximately 3.5. It has a pearly luster on cleavage surfaces and a vitreous luster on other faces. It is transparent to translucent. The density of walthierite has been calculated at 3.13 g/cm³. ## Colors and Varieties Walthierite is colorless or white. No colored or commercial varieties have been distinguished. ## History and Name The mineral was discovered in the Golden Queen mine in the Mojave Mountains, Kern County, California (USA). It was described and approved as a new mineral species by the IMA in 1992. The name honors Thomas N. Walthier (1923–1989), an American geologist and exploration manager, in recognition of his contributions to mineral deposit exploration. ## Applications Due to its rarity and microscopic crystal size, walthierite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals or minerals of the alunite group.

Properties

Mohs hardness
3.5
Luster
Pearly, Vitreous
Streak
White
Density
3.13
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent to Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Field identification of walthierite is practically impossible due to the microscopic size of its crystals. In a collection, it can be recognized by its characteristic habit of small, hexagonal, platy crystals, often in rosette form, and its pearly luster. Definitive confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). ## Distinguishing from Similar Minerals Walthierite is visually almost identical to other minerals of the alunite group, such as alunite, natroalunite, or huangite. Certain distinction from these minerals is possible only based on chemical composition analysis, which will show the presence and dominance of barium. ## Crystal Forms Walthierite crystals are platy, with a hexagonal or pseudohexagonal outline. They typically form small, rosette-like or lamellar aggregates and clusters on the surface of the host rock.

Geological environment

## Genesis Walthierite is a mineral of hydrothermal origin. It forms as a result of the interaction of acidic, sulfate-rich solutions with volcanic rocks, such as rhyolite or dacite. It is a product of advanced argillization (alteration to clay minerals) under low-temperature conditions. ## Mineral Associations This mineral co-occurs with other minerals typical of hydrothermal alteration zones. It is most commonly associated with: alunite, quartz, kaolinite, dickite, pyrophyllite, diaspore, svanbergite, woodhouseite, and sulfides such as pyrite. ## Localities The most important and type locality for walthierite is the Golden Queen mine in Soledad Mountain, Kern County, California, USA. It has also been identified in other locations worldwide, including the Marysvale mine in Utah (USA) and the volcanic complex on La Gomera island (Canary Islands, Spain).

Rarity

Very rare

For collectors

## Quality Criteria For walthierite collectors, the most important criterion is the quality and abundance of crystals on the rock matrix. Specimens with well-formed, sharp, platy crystals forming dense aggregates or rosettes are most valued. Contrast with the color of the host rock also enhances the specimen's attractiveness. Due to the microscopic nature of the mineral, the size of individual crystals, even if sub-millimeter, is significant. ## Popular Localities By far the most desired and classic walthierite specimens come from the type locality – the Golden Queen mine in California. Specimens from this location are the standard for this mineral.

Care and storage

## Cleaning Walthierite specimens, due to their delicacy and small size, should be cleaned with utmost care. The safest method is to use compressed air to remove dust. If wet cleaning is necessary, use only distilled water and a very soft brush, avoiding any mechanical pressure. ## What to Avoid Avoid contact with acids and other chemicals that can damage the mineral. Walthierite is a relatively soft mineral, making it susceptible to scratches. Protect it from ultrasound, high temperatures, and sudden temperature changes. ## Storage It is recommended to store walthierite specimens in closed, padded "micromount" boxes to protect them from dust, mechanical damage, and contact with other minerals. Display should be away from direct sunlight and heat sources.

Sources

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