Betpakdalite-NaCa
Chemical formula: [Na<sub>2</sub>(H<sub>2</sub>O)<sub>17</sub>Ca(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>34</sub>(OH)<sub>3</sub>]
Betpakdalite-NaCa is a very rare arsenate-iron molybdate, forming microscopic, yellow crystals with a hexagonal outline.
Properties
- Mohs hardness
- 2
- Luster
- Vitreous
- Streak
- Yellow
- Density
- 3.25
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification The mineral can be identified by its characteristic, intensely yellow color, specific form of small, hexagonal tablets, and occurrence in the oxidation zones of molybdenum deposits. It is very soft and often forms coatings on other minerals. ## Distinguishing from Similar Minerals It can be confused with other secondary molybdenum minerals, such as wulfenite or molybdofornacite, but differs from them in crystal habit and significantly lower hardness. Wulfenite forms larger, square crystals, while molybdofornacite typically has greenish or brownish hues. Final identification requires advanced analytical methods (XRD, EDS). ## Crystal Forms Crystals are most often thin-tabular, with a distinct hexagonal outline. They form rosette-like or radial aggregates, as well as thin coatings and earthy efflorescences.
Geological environment
## Genesis Betpakdalite-NaCa is a secondary mineral, forming in the oxidation (weathering) zones of hydrothermal deposits rich in molybdenum, arsenic, and iron. It forms as a result of surface water action on primary sulfides and arsenides under low temperature and pressure conditions. ## Mineral Associations It often co-occurs with other minerals of the oxidation zone, such as quartz, jarosite, goethite, hematite, molybdenite, pyrite, as well as other rare molybdates and arsenates. ## Localities The most important and classic localities for this mineral are in Kazakhstan, in the Betpak-Dala Desert (Kara-Oba mine) and in Kyrgyzstan. It is also known from several localities in the USA (e.g., T.N. mine in Nevada) and other places worldwide where oxidized molybdenum deposits occur.
Rarity
Very rare
For collectors
## Quality Criteria Most valued by collectors are specimens with well-formed, sharp crystals of intense, golden-yellow color, forming aesthetic, rosette-like aggregates on a contrasting rock matrix. Due to the microscopic size of the crystals, their quality and form visible under magnification are crucial. The size of the aggregates and the abundance of occurrence on the matrix also increase the specimen's value. ## Popular Localities Specimens from the type locality in Kazakhstan (Kara-Oba) are historically most important. Currently, specimens from Kyrgyzstan and the United States (Nevada) also appear on the collector's market and are sometimes valued for crystal quality.
Care and storage
## Cleaning Specimens should be cleaned only very carefully, using compressed air to remove dust. Due to its extreme delicacy and solubility, any contact with water or other liquids should be avoided. Brushes or ultrasonics must not be used. ## What to Avoid Contact with water, acids, and other chemicals that can dissolve or damage the mineral must be absolutely avoided. It is sensitive to high temperatures, which can cause dehydration and destruction. It should be protected from direct sunlight and humidity. ## Storage Specimens should be stored in dry conditions, preferably in tightly sealed containers (such as "micromount boxes") away from sources of vibration and temperature changes. Due to its fragility, touching the crystals should be avoided.