Hydroxylborite

Chemical formula: Mg<sub>3</sub>(BO<sub>3</sub>)(OH)<sub>3</sub>

Magnesium borate, forming very rare, white, earthy or chalky aggregates in skarns.

## Characteristics Hydroxylborite is a hydrated magnesium borate, belonging to the group of rare minerals. It occurs almost exclusively as very fine-grained, compact aggregates resembling chalk or clay. Its aggregates can be porous and dull. Individual crystals are microscopic, in the form of lamellae or needles, and form radial or spherulitic (spherical) structures, visible only under high magnification. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2, meaning it can be scratched with a fingernail. It has a low density, about 2.55 g/cm³. It is opaque, and its luster is described as dull or earthy. Due to its microcrystalline and often porous structure, specimens are brittle and light. ## Colors and Varieties Hydroxylborite is a mineral of uniform, white color. It does not form color or commercial varieties. ## History and Name The mineral's name, given in 1968, directly refers to its chemical composition – the presence of hydroxyl groups and boron. It was first identified and described based on samples from the Suan mine in North Hwanghae Province, North Korea. ## Applications Due to its extreme rarity and occurrence only in small aggregates, hydroxylborite has no industrial application. It is solely an object of scientific interest and a valued acquisition in advanced collections of rare minerals and microminerals.

Properties

Mohs hardness
2
Streak
White
Density
2.55
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Characteristic features of hydroxylborite include its white color, earthy or chalky appearance, very low hardness (about 2 on the Mohs scale), and specific occurrence environment (magnesian skarns). In field conditions, it is practically impossible to distinguish from other white, earthy secondary minerals. Certain identification requires advanced analytical methods, such as X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Hydroxylborite can be confused with szaibelyite, with which it often co-occurs and which has a similar appearance and chemical composition. Distinguishing these two minerals requires laboratory analysis. It can also be confused with other white, soft secondary minerals, such as hydromagnesite or earthy talc. A key diagnostic clue is its association with other borates, e.g., ludwigite. ## Crystal Forms Macroscopically, it forms only earthy, chalky, or compact aggregates. Under the microscope, it reveals a structure composed of very small, lamellar or acicular crystals, which often arrange into spherulitic (spherical) or radial aggregates.

Geological environment

## Genesis Hydroxylborite is a secondary mineral, forming in hydrothermal alteration zones of magnesian skarns. It forms as a result of the alteration of earlier, anhydrous magnesium borates, such as kotoite, under conditions of elevated water activity. It is a product of serpentinization processes of ultrabasic rocks rich in boron. ## Mineral Associations It most often co-occurs with other magnesium borates, such as szaibelyite (often in direct intergrowths), ludwigite, and kotoite. It is also accompanied by rock-forming minerals of skarns and serpentinites, including serpentine, magnetite, brucite, calcite, and dolomite. ## Localities The most important and classic locality of hydroxylborite, from which the best-known specimens originate, is its type locality – the Suan gold and copper mine in North Korea. It is a very rare mineral, reported only from a few places in the world with similar geology, including some boron deposits in Siberia, Russia.

Rarity

Very rare

For collectors

## Quality Criteria As a mineral occurring exclusively in microcrystalline form, hydroxylborite is primarily valued by collectors specializing in rare species and microminerals. The value of a specimen is determined primarily by the certainty of its identification (analytically confirmed), the richness of the aggregate on the rock matrix, and the presence of well-formed associated minerals. Samples from the type locality (Suan mine) are most highly prized. ## Popular Localities The only classic and widely known source of collector specimens is the Suan mine in North Korea.

Care and storage

## Cleaning Hydroxylborite specimens are extremely delicate and soft. To remove dust, use only a soft brush or a stream of compressed air from a safe distance. Avoid any contact with water and other liquids. ## What to Avoid The mineral is sensitive to moisture and can be damaged by contact with water. Ultrasonic cleaners, all chemicals, acids, and detergents should be strictly avoided. Protect from impacts and scratching due to its very low hardness. ## Storage It is recommended to store specimens in closed, padded "micromount" boxes to protect them from mechanical damage, dust, and moisture. Store in a dry place.

Sources

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