Hydroxycalciomicrolite

Chemical formula: Ca<sub>1.5</sub>Ta<sup>5+</sup><sub>2</sub>O<sub>6</sub>(OH)

Hydroxycalciomicrolite is a rare mineral from the microlite group, a tantalum and calcium oxide, found as small, octahedral crystals.

## Characteristics Hydroxycalciomicrolite is a mineral belonging to the microlite group within the pyrochlore supergroup. It occurs in the form of very small, well-formed, octahedral crystals, whose size rarely exceeds 1 mm. The crystals are typically isometric and have a characteristic, sharp outline. ## Physical Properties This mineral is characterized by a vitreous luster, and on fracture surfaces, a greasy or resinous luster. It is brittle, and its Mohs hardness is about 5. The density is high, which is typical for tantalum-bearing minerals, and is about 5.3 g/cm³. ## Colors and Varieties Hydroxycalciomicrolite is usually colorless, white, pale yellow, or light brown. No specific color varieties or commercial forms are distinguished. ## History and Name The mineral's name refers to its chemical composition – it indicates the dominance of the hydroxyl group (OH), calcium (calcio), and its belonging to the microlite group. It was formally described and recognized by the International Mineralogical Association (IMA) in 2013, and its discovery is associated with research on minerals from the pyrochlore supergroup. ## Uses Due to its rarity and microscopic crystal sizes, hydroxycalciomicrolite has no industrial applications. It is solely an object of scientific and collecting interest for specialists gathering systematic minerals or microminerals.

Properties

Mohs hardness
5
Luster
Vitreous to resinous
Streak
White
Density
5.3
Cleavage
Imperfect on {111}
Fracture
Conchoidal to uneven
Transparency
Transparent to translucent
Crystal system
Cubic

Diagnostic features

## Identification Identification of hydroxycalciomicrolite under amateur conditions is practically impossible. It requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS) to confirm the chemical composition, especially the calcium-to-tantalum ratio and the presence of the hydroxyl group. The characteristic octahedral habit of the crystals is an indication but not a sufficient feature for identification. ## Distinguishing from Similar Minerals This mineral is visually indistinguishable from other members of the microlite group, such as fluorcalciomicrolite, oxycalciomicrolite, or microlite itself. All form similar, isometric crystals of similar colors. Certain differentiation is possible only based on detailed chemical analysis. ## Crystal Forms Hydroxycalciomicrolite crystallizes in the isometric system, forming almost exclusively crystals with an octahedral habit {111}. Crystals are usually single, grown on the surface of the matrix rock.

Geological environment

## Genesis Hydroxycalciomicrolite is a mineral of magmatic origin, crystallizing in the late stages of evolution of granitic pegmatites, especially those of the LCT (lithium-cesium-tantalum) type. It forms under conditions of enrichment in rare elements, such as tantalum, and in the presence of water, which allows the incorporation of hydroxyl groups into its structure. ## Mineral Associations This mineral co-occurs with other minerals typical of granitic pegmatites, such as quartz, albite, microcline, muscovite, lepidolite, as well as with other tantalum and niobium minerals, e.g., columbite-(Mn), tantalite-(Mn), and other representatives of the microlite group. ## Localities The most important and best-documented occurrence of hydroxycalciomicrolite, which is also its type locality, is the Volta Grande pegmatite in the municipality of Nazareno, Minas Gerais state, Brazil.

Rarity

Very rare

For collectors

## Quality Criteria The collecting attractiveness of hydroxycalciomicrolite specimens primarily depends on the quality and size of the crystals, which in this case means specimens exceeding 0.5 mm. The most valued are sharp, well-formed, undamaged octahedra with a distinct luster, contrastingly set on the rock matrix. Transparency and clarity of the crystals also increase their value. ## Popular Localities The only source of well-characterized collector specimens is the type locality – the Volta Grande pegmatite in Brazil. Specimens from this location are the standard for this mineral.

Care and storage

## Cleaning Specimens should only be cleaned with compressed air to remove dust. Due to the microscopic size of the crystals, mechanical cleaning (e.g., with a brush) or the use of liquids is not recommended and can lead to damage or loss of crystals. ## What to Avoid Avoid contact with chemicals, especially acids, which can react with the mineral. Also, avoid ultrasonic cleaners, which can destroy delicate crystals or detach them from the matrix rock. ## Storage Hydroxycalciomicrolite specimens, as microminerals, should be stored in specialized "micromount" boxes that protect them from dust, shocks, and mechanical damage. Display should be done under a microscope.

Sources

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