Metaborite

Chemical formula: HBO<sub>2</sub>

Metaborite is a rare borate mineral, a polymorph of metaboric acid, occurring as tiny, colorless crystals.

## Characteristics Metaborite is a mineral from the borate group, chemically corresponding to metaboric acid (HBO₂). It occurs as very small, tabular or prismatic crystals, rarely exceeding 1 mm in length. It typically forms colorless, white, or grayish aggregates and coatings. It is one of three natural polymorphs of HBO₂, alongside clinometaborite and hexagonal γ-HBO₂. ## Physical Properties Metaborite crystals exhibit a vitreous luster. This mineral is relatively soft, with a Mohs hardness of 3. It is transparent to translucent. Due to the small size of the crystals, its physical properties are difficult to study on macroscopic specimens. ## Colors and Varieties Metaborite is usually colorless or white. No colored or commercial varieties are distinguished. ## History and Name The mineral's name refers to its chemical composition – it is the natural analogue of metaboric acid. It was first described in 1962 by E. V. Kondrat'eva based on material found in drill cores from the Inder region in Kazakhstan. ## Uses Metaborite has no industrial application. It is of purely scientific significance and is sought after by specialized collectors of rare minerals.

Properties

Mohs hardness
3
Luster
Vitreous
Streak
White
Density
2.47
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Cubic

Diagnostic features

## Identification Metaborite is difficult to identify visually due to the small size of its crystals. It is primarily recognized based on its occurrence environment – in association with other borates in salt deposits. A key feature is its reactivity with water. Confirmation of identity requires advanced analytical methods, such as X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other colorless or white borates, such as sassolite, borax, or colemanite, with which it often co-occurs. It is distinguished from them by its habit of small, tabular crystals and its solubility in water. Unlike sassolite, it is not as distinctly platy and greasy to the touch. ## Crystal Forms It forms very small, tabular or short-prismatic crystals with a square or rectangular cross-section. It occurs as single, dispersed crystals or fine-grained aggregates and coatings.

Geological environment

## Genesis Metaborite is an evaporite mineral. It forms as a result of dehydration of sassolite (H₃BO₃) in dry, saline environments, such as drying borate lakes and salt flats. It can also form as a product of volcanic exhalations (fumaroles). ## Mineral Associations It most commonly co-occurs with other borates, such as sassolite, borax, ulexite, colemanite, as well as halite and gypsum. ## Localities The most important localities worldwide include the Inder region in Kazakhstan (type locality), where it occurs in the gypsum caps of salt domes. It is also known from the Death Valley region in California (USA) and from geothermal fields in Larderello, Italy, where it forms as a result of fumarolic activity.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of metaborite primarily depends on the presence of well-formed, even microscopic, crystals on the rock matrix. Due to its rarity, any analytically confirmed specimen has value for specialized collectors. A contrasting substrate and rich association with other borates enhance the visual and scientific value of the specimen. ## Popular Localities The most prized specimens, though still microscopic, come from classic localities such as the Inder region in Kazakhstan and some localities in California, USA.

Care and storage

## Cleaning Metaborite specimens are sensitive to water and moisture. Cleaning should only be done dry, using a soft brush or compressed air to remove dust. All contact with liquids should be avoided. ## What to Avoid Contact with water, water vapor, and high air humidity must be absolutely avoided, as metaborite is soluble and can be destroyed. It should also be protected from chemicals and high temperatures. ## Storage Storage requires airtight conditions. It is best to place the specimen in a sealed container (e.g., a "perky box") along with a moisture-absorbing agent (e.g., silica gel). Display should be in a closed display case, away from sources of moisture.

Sources

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