Pentahydroborite
Chemical formula: CaB<sub>2</sub>O(OH)<sub>6</sub>·2H<sub>2</sub>O
Pentahydroborite is a rare hydrated calcium borate, forming colorless, acicular crystals in salt deposits.
Properties
- Mohs hardness
- 2.5
- Luster
- Vitreous, Silky
- Streak
- White
- Density
- 1.99-2.02
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Pentahydroborite most often forms white, radial or tangled aggregates of small, acicular crystals with a silky luster. Low hardness (approximately 2.5 on the Mohs scale) and reaction to water are key features. It occurs in the specific environment of salt evaporites. ## Distinguishing from Similar Minerals It can be confused with other white, fibrous borates or sulfates found in similar environments, such as ulexite or colemanite. Ulexite often exhibits a characteristic "television effect" (image conduction), which is absent in pentahydroborite. Accurate differentiation from other rare borates requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Crystals are elongated, acicular or fibrous, often gathered in radial, stellate, or haphazardly tangled aggregates. Single, well-formed crystals are rarely observed.
Geological environment
## Genesis Pentahydroborite is a mineral of sedimentary origin. It forms as a result of the evaporation of waters in closed, saline sedimentary basins (borate lakes), in a dry and hot climate. ## Mineral Associations It most commonly co-occurs with other borate and salt minerals, such as hydroboracite, colemanite, ulexite, inyoite, as well as gypsum and halite. ## Localities The most important and classic occurrences of this mineral are found in Kazakhstan, in the Atyrau region (e.g., the Inder boron deposit). It has also been reported in other localities with similar geology, such as Death Valley in California (USA).
Rarity
Rare
For collectors
## Quality Criteria The collector's appeal of pentahydroborite primarily depends on the richness and aesthetics of the aggregates. Specimens with well-defined, radial clusters of acicular crystals, forming contrasting growths on the host rock, are most valued. The size and purity (lack of impurities) of the aggregates also increase the specimen's value. Due to its rarity, every well-documented specimen has value for specialized collectors. ## Popular Localities By far the most known and valued specimens come from the boron deposits in the Inder region of Kazakhstan. Material from this locality is considered reference for this mineral.
Care and storage
## Cleaning Pentahydroborite specimens are very delicate and sensitive to water. Cleaning should be kept to an absolute minimum. A soft, dry brush can be used to remove dust. Any contact with water, even distilled, risks dissolving or damaging the mineral. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is sensitive to changes in humidity and temperature. It should not be heated or exposed to direct sunlight. ## Storage Store in stable conditions, in a dry place, preferably in a tightly sealed display box away from sources of moisture. Due to its fragility and low hardness, it should be stored separately to avoid abrasion by harder minerals.