Betpakdalite-NaNa

Chemical formula: [Na<sub>2</sub>(H<sub>2</sub>O)<sub>16</sub>Na(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>33</sub>(OH)<sub>4</sub>]

Betpakdalite-NaNa is a very rare molybdate of the betpakdalite group, forming microscopic, yellow crystals in spherical aggregates.

## Characteristics Betpakdalite-NaNa is a hydrated sodium iron arsenate-molybdate. This mineral forms very small, acicular or bladed crystals, which usually occur as radial or spherical aggregates. Individual crystals rarely exceed 0.2 mm in length. Due to its size, its visual characteristics are most often observed under a microscope. ## Physical properties Betpakdalite-NaNa crystals exhibit a silky luster. Hardness and density have not been precisely determined due to the small size and rarity of the crystals. It is a brittle mineral. ## Colors and varieties This mineral occurs in a characteristic canary-yellow color. No color or commercial varieties are known. ## History and name Betpakdalite-NaNa was first described in 2012 by J. Plášek and collaborators. Its name refers to the original mineral of the group – betpakdalite – and the dominant sodium (Na) ions in two different positions in its crystal structure. The group name comes from the discovery location of the first mineral in this series, the Betpak-Dala Desert in Kazakhstan.

Properties

Mohs hardness
2
Luster
Silky
Streak
Yellow
Density
3.16
Cleavage
Perfect on {010}
Fracture
Splintery
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of betpakdalite-NaNa is possible almost exclusively using advanced analytical methods, such as X-ray diffraction (XRD) and Raman spectroscopy. Visually, the occurrence of canary-yellow, spherical aggregates in paragenesis with other rare molybdates may raise suspicion. ## Distinguishing from similar minerals It can be confused with other yellow secondary molybdenum and arsenic minerals, such as other minerals from the betpakdalite group, wulfenite, or ferrimolybdite. Certain distinction requires specialized chemical and structural analysis, as they are often visually indistinguishable, especially in microcrystalline form. ## Crystal forms It forms very fine, acicular crystals grouped into spherical or radial aggregates. Less commonly, individual, tabular crystals are observed.

Geological environment

## Genesis Betpakdalite-NaNa is a secondary mineral, formed in the oxidation zones (gossans) of polymetallic deposits rich in molybdenum and arsenic. It crystallizes from aqueous solutions under conditions of low temperature and pressure. ## Mineral associations It co-occurs with other rare secondary minerals. In the type locality (Blue Lizard mine), it was found in association with quartz, barite, gypsum, jarosite, natrojarosite, hematite, as well as other minerals from the betpakdalite group, such as betpakdalite-CaCa and betpakdalite-NaCa. ## Localities The only confirmed and thoroughly described occurrence of this mineral in the world is the Blue Lizard mine in San Juan County, Utah, USA. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality criteria For such a rare "micromount" mineral, the main criterion of value is the confirmation of its identity by a reputable laboratory. From a collector's perspective, specimens where betpakdalite-NaNa aggregates are best formed, clearly visible on the rock matrix, and abundant are most desirable. Contrast with associated minerals also enhances visual appeal.

Care and storage

## Cleaning Due to its extreme rarity and delicacy, specimens of this mineral are practically not subject to cleaning. Any contaminants should be left undisturbed. If absolutely necessary, compressed air can be used from a safe distance. ## What to avoid Avoid contact with water, chemicals, ultrasound, and all forms of mechanical cleaning. The crystals are very brittle and sensitive to changes in humidity and temperature. ## Storage Specimens should be stored only in specialized, sealed "micromount" containers, protecting them from vibrations, dust, and sudden environmental changes. It is not suitable for display outside such a container.

Sources

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