Betpakdalite-CaMg

Chemical formula: [Ca<sub>2</sub>(H<sub>2</sub>O)<sub>17</sub>Mg(H<sub>2</sub>O)<sub>6</sub>][Mo<sup>6+</sup><sub>8</sub>As<sup>5+</sup><sub>2</sub>Fe<sup>3+</sup><sub>3</sub>O<sub>36</sub>(OH)]

Betpakdalite-CaMg is a very rare arsenate, molybdate, and oxide, forming microscopic, yellow crystals in spherical aggregates.

## Characteristics Betpakdalite-CaMg is a complex oxide, arsenate, and molybdate of calcium, magnesium, and iron. It occurs as very small, acicular or bladed crystals, which most often form spherical or radial aggregates. Individual crystals rarely exceed fractions of a millimeter in length, and entire aggregates reach sizes of up to several millimeters. This mineral is valued by collectors specializing in rare species and micromounts. ## Physical Properties Crystals exhibit a vitreous luster. Due to the microscopic size of the crystals, most physical properties, such as hardness or density, have not been precisely determined for individual specimens. ## Colors and Varieties Betpakdalite-CaMg is characterized by an intense, canary-yellow color. No color varieties or commercial names are known. ## History and Name The mineral was approved by the IMA in 2011. Its name refers to the original mineral of this group - betpakdalite (now Betpakdalite-NaCa), discovered in the Betpak-Dala desert in Kazakhstan, and to the dominant cations in its composition: calcium (Ca) and magnesium (Mg).

Properties

Luster
Vitreous
Streak
Yellow
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification The mineral can be identified by its characteristic canary-yellow color, its occurrence as small, spherical or radial aggregates, and its specific vitreous luster. Its occurrence in the oxidation zones of polymetallic deposits is also key. ## Differentiation from Similar Minerals Betpakdalite-CaMg can be confused with other secondary molybdenum and arsenic minerals of yellow color, such as Wulfenite or Mimetite. Distinguishing it requires advanced analytical methods, such as Raman spectroscopy or EDS analysis, due to the microscopic size of the crystals. From other minerals of the betpakdalite group (e.g., Betpakdalite-NaCa), it differs in chemical composition, which is impossible to determine without specialized equipment. ## Crystal Forms Crystals are very small, acicular or bladed. They typically form spherical, radial aggregates, as well as bundles or fan-like clusters.

Geological environment

## Genesis Betpakdalite-CaMg is a secondary mineral, forming in the oxidation zones (so-called iron caps or gossans) of polymetallic deposits that are rich in arsenic and molybdenum. It forms as a result of the weathering of primary sulfides and molybdates in the presence of calcium- and magnesium-rich waters. ## Mineral Associations This mineral coexists with other secondary minerals of the oxidation zone, such as Quartz, Goethite, Jarosite, and also with other arsenates and molybdates. ## Localities The only confirmed and described occurrence of this mineral in the world is the Tsumeb mine in Namibia, which is its type locality. This is a historic mine known for its extraordinary richness and diversity of rare secondary minerals.

Rarity

Extremely rare

For collectors

## Quality Criteria The most prized specimens are those where the spherical aggregates of Betpakdalite-CaMg are well-formed, have an intense, canary-yellow color, and are set on a contrasting substrate (rock matrix). Due to the microscopic nature of the mineral, the size of the aggregates (even up to several millimeters) significantly increases the value of the specimen. Purity here refers mainly to the absence of damage to the delicate, acicular crystals. ## Popular Localities The only source of collector specimens is the historic Tsumeb mine in Namibia. Material from this locality is extremely rare and sought after by specialized collectors.

Care and storage

## Cleaning Specimens should only be cleaned with compressed air to remove dust. Due to extreme delicacy and reactivity, any contact with water, chemical solutions, or even alcohol should be avoided. Mechanical cleaning (e.g., with a brush) is impossible and will lead to the destruction of the specimen. ## What to Avoid Contact with water and chemicals, which can dissolve or damage the microscopic crystals, must be strictly avoided. The mineral is sensitive to changes in temperature and humidity. It should not be heated or exposed to direct sunlight, which can cause it to fade. ## Storage Betpakdalite-CaMg specimens should be stored exclusively in specialized "micromount" boxes that protect them from dust, mechanical damage, and sudden changes in humidity. Storage in a dry place is crucial for preserving the integrity of the specimen.

Sources

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