Hydroboracite

Chemical formula: CaMg[B<sub>3</sub>O<sub>4</sub>(OH)<sub>3</sub>]<sub>2</sub>·3H<sub>2</sub>O

A hydrous calcium magnesium borate, forming acicular or fibrous aggregates with a silky luster, often in the form of radiating spherical clusters.

## Characteristics Hydroboracite is a hydrous calcium magnesium borate belonging to the borate group. Its most typical form of occurrence is radiating or fibrous aggregates, which can form spherical or hemispherical clusters. Individual crystals are rare, usually appearing as elongated, acicular or bladed crystals with a square cross-section. These aggregates often exhibit a characteristic silky luster, resulting from their fibrous structure. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3.5. It is light, with a density of approximately 2.17 g/cm³. Crystals are brittle and easily damaged. It exhibits perfect cleavage in one direction, parallel to the crystal elongation. It is transparent to translucent, and its luster is described as vitreous on crystal faces and silky on fracture surfaces and in fibrous aggregates. ## Colors and Varieties Hydroboracite is usually colorless or white. It can take on shades of gray, yellowish, or pinkish due to impurities and inclusions. There are no named color varieties or commercial varieties. ## History and Name The mineral's name, given in 1834 by Germain Henri Hess, refers to its chemical composition – the presence of water (hydro) and boron (boracite). It was discovered in the Caucasus. ## Uses Due to its boron content, hydroboracite can be locally used as an ore of this element if it occurs in sufficiently large and rich deposits. For collectors, it is interesting due to its attractive, radiating aggregate forms.

Properties

Mohs hardness
3.5
Luster
Vitreous, Silky
Streak
White
Density
2.16 - 2.17
Cleavage
Perfect on {100}, Good on {010}
Fracture
Uneven, Splintery
Transparency
Transparent to Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification A characteristic feature of hydroboracite is its radiating, fibrous aggregates with a silky luster. They often form spherical clusters (spherulites). It reacts with hydrochloric acid, releasing heat. It is partially soluble in hot water. Under a blowpipe flame, it swells and melts, forming a transparent glass. ## Distinguishing from Similar Minerals It can be confused with other borates of similar form, such as colemanite or ulexite. Ulexite often exhibits the "television stone" effect (image transmission), which is absent in hydroboracite. Colemanite is harder (4.5 on the Mohs scale) and has a different crystal habit. Other fibrous minerals, like gypsum (selenite variety), are much softer (hardness 2). ## Crystal Forms Crystals are monoclinic, but often elongated and acicular, giving them a pseudo-tetragonal appearance. Most commonly found in the form of radiating-fibrous aggregates, rosettes, spherulitic clusters, and compact, felted masses.

Geological environment

## Genesis Hydroboracite is an evaporite mineral. It forms as a result of the evaporation of saline lake waters (playas) in arid, desert climates. It crystallizes in boron-rich lake sediments, often in clays or marls, either directly from solutions or through the alteration of other, previously formed borate minerals. ## Mineral Associations It co-occurs with other borate minerals such as colemanite, ulexite, inyoite, borax, and kernite. It is also often accompanied by halite, gypsum, anhydrite, and calcite. ## Localities Significant deposits and beautiful collector specimens come from Kazakhstan (Inder deposit). It also occurs in Turkey (Balıkesir province), the United States (Death Valley and Inyo County in California; Clark County in Nevada), Argentina (Salta and Jujuy provinces), and China.

Rarity

Not very common

For collectors

## Quality Criteria The most prized by collectors are well-formed, spherical or hemispherical radiating aggregates with a distinct silky luster. The size of these "spheres" and their completeness and lack of damage are important. Specimens on a rock matrix are usually more desirable than individual, loose aggregates. Purity (lack of contamination and clay inclusions) also increases the value of the specimen. ## Popular Localities The most famous collector specimens, known for large and well-formed spherulites, come from the Inder boron deposit in Kazakhstan. High-quality material is also supplied by localities in California (USA) and Turkey.

Care and storage

## Cleaning It should only be cleaned dry, using a soft brush to remove dust. Contact with water is absolutely inadvisable, as the mineral is partially soluble in it and may become dull or damaged. ## What to Avoid Avoid all contact with water, acids, and other chemicals. The mineral is sensitive to changes in humidity and temperature. It should not be heated or exposed to direct sunlight, which can lead to its dehydration and destruction. ## Storage Hydroboracite specimens are best stored in closed, dry containers or display cases to protect them from moisture and dust. Due to its brittleness and low hardness, it should be stored separately from harder minerals to avoid scratches and mechanical damage.

External references

Sources

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