Pentahydroborite is a rare hydrated calcium borate mineral that typically forms delicate, needle-like acicular crystal clusters. It is most commonly found as white, glassy sprays coating the walls of cavities within borate-rich sedimentary deposits. Due to its scarcity and fragile nature, it is primarily sought after by advanced systematic mineral collectors.
Is this pentahydroborite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch pentahydroborite with a known reference. Pentahydroborite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: colorless.
- 5Look at form & habitCrystal system: triclinic. Typical habit: granular.
Often confused with
Pentahydroborite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside pentahydroborite
Minerals reported to co-occur with pentahydroborite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaB₂O₄·5H₂O
- Mohs hardness
- 2.5
- Density
- 2.00-2.03 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Granular
- Cleavage
- None
- Fluorescence
- Violet under SW UV
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In boron-rich iron-ore skarns (Novofrolovskoye deposit, Russia); in a volcanogenic-sedimentary borate deposit (near the Studenitsa Cloister, Yugoslavia)
Where rockhounds find pentahydroborite
Classic worldwide localities
- Novofrolovskoye copper deposit, near Krasnoturinsk, Turinsk district, Northern Ural Mountains, and at the Solongo boron deposit, Buryatia, Siberia, Russia
- Sayak-IV boron deposit, northeast Balkhash region, Kazakhstan
- Fuka, near Bicchu, Okayama Prefecture, Japan
Field-hunting tip
Where it forms: In boron-rich iron-ore skarns (Novofrolovskoye deposit, Russia); in a volcanogenic-sedimentary borate deposit (near the Studenitsa Cloister, Yugoslavia). If you start seeing andradite–grossular, magnetite, szaibélyite in float, you are in the right ground. Typical habit: granular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify pentahydroborite?+
Where is pentahydroborite found?+
What rocks look like pentahydroborite?+
What minerals are found with pentahydroborite?+
What kind of rock does pentahydroborite form in?+
What is pentahydroborite used for?+
Find pentahydroborite on the map
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