Oxybismutomicrolite

Chemical formula: (Bi<sup>3+</sup><sub>1.33</sub>&#9744;<sub>0.67</sub>)Ta<sup>5+</sup><sub>2</sub>O<sub>6</sub>O

Oxybismutomicrolite is a very rare bismuth and tantalum oxide from the microlite group, found as small, resinous-brown grains in granitic pegmatites.

## Characteristics Oxybismutomicrolite is an extremely rare mineral from the microlite group, belonging to the pyrochlore supergroup. Chemically, it is a bismuth and tantalum oxide. It occurs as very small, irregular grains or poorly formed, octahedral crystals, usually not exceeding 0.5 mm in size. Its grains are most often intergrown with other minerals, mainly quartz or albite. ## Physical Properties This mineral is characterized by a resinous to almost metallic luster. It is brittle, and its Mohs hardness is approximately 5. The density is very high, which is typical for minerals containing tantalum and bismuth, and is approximately 7.43 g/cm³. ## Colors and Varieties Oxybismutomicrolite has a characteristic brown to reddish-brown color. It is translucent to almost opaque. No varieties have been distinguished. ## History and Name The mineral's name refers to its chemical composition – the dominance of oxygen (Oxy) and bismuth (bismuto) and its belonging to the microlite group (microlite). It was formally described and approved by the International Mineralogical Association (IMA) in 2019. It was discovered in material from the Solnechnaya pegmatite in the Ulan-Salga Mountains in Tuva, Russia.

Properties

Mohs hardness
5
Luster
Resinous to metallic
Streak
Light brownish
Density
7.43
Cleavage
None
Fracture
Uneven to subconchoidal
Transparency
Translucent to opaque
Crystal system
Isometric

Diagnostic features

## Identification Identification of oxybismutomicrolite is impossible without advanced analytical methods, such as energy-dispersive X-ray spectroscopy (EDS) or X-ray diffraction (XRD). Preliminary field identification is practically unfeasible. In a collection, it might be suspected based on high density, resinous luster, brown color, and co-occurrence with pegmatitic minerals. ## Distinguishing from Similar Minerals It can be confused with other minerals from the pyrochlore group, such as microlite, pyrochlore, or other tantalum and niobium oxides (e.g., cassiterite, rutile). Definitive distinction requires specialized chemical analysis to confirm the dominance of bismuth and tantalum. ## Crystal Forms It most often forms anhedral (irregular) grains or subhedral (poorly formed) octahedral crystals, up to 0.5 mm in size.

Geological environment

## Genesis Oxybismutomicrolite forms in the late stages of crystallization of lithium-cesium-tantalum (LCT-type) granitic pegmatites, rich in bismuth. It is a mineral of hydrothermal or pneumatolytic phase. ## Mineral Associations This mineral co-occurs with quartz, albite, microcline, lepidolite, elbaite, cassiterite, manganotantalite, native bismuth, and other bismuth minerals. ## Localities The only confirmed occurrence (type locality) is the Solnechnaya pegmatite, located in the Ulan-Salga mountain range, in the Republic of Tuva, Russia.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of oxybismutomicrolite specimens is primarily assessed based on the confirmation of its identity through analysis. The most valuable specimens are those with the largest possible, well-formed crystals, although even microscopic grains have great scientific and collector value. Contrast with the surrounding host rock (matrix) also enhances the specimen's attractiveness. ## Popular Localities The only source of specimens is the type locality in Russia. Material from this location is extremely difficult to obtain and appears on the collector's market sporadically.

Care and storage

## Cleaning Due to its extreme rarity and small grain size, specimens with oxybismutomicrolite practically do not require cleaning. If necessary, compressed air can be used to remove dust. Contact with water and chemical cleaning agents should be avoided. ## What to Avoid Ultrasonic cleaners, sudden temperature changes, and all acids and bases that could damage this delicate mineral or its host rock should be avoided. ## Storage Specimens should be stored under stable conditions, in closed display boxes, to protect them from dust and mechanical damage. Due to the small size of the crystals, storage in "micromount" type boxes is recommended.

Sources

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