Tunellite is a rare strontium borate mineral that typically forms as delicate, transparent, tabular crystals or radiating clusters. It is primarily found in California borate deposits, often associated with other rare borate species in evaporite sequences.
Is this tunellite?
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 tunellite with a known reference. Tunellite 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: subvitreous, pearly on {100} faces and cleavages.
- 4Match the color rangeCompare against the expected color range: colorless.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: nodular, prismatic, tabular.
Often confused with
Tunellite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Colemanite is the harder of the two (Mohs 4.5 vs. 2.5); luster reads subvitreous, pearly on {100} faces and cleavages on Tunellite and vitreous to adamantine on Colemanite.
How to tell apart: Luster reads subvitreous, pearly on {100} faces and cleavages on Tunellite and vitreous; silky or satiny in fibrous aggregates on Ulexite.
Often found alongside tunellite
Minerals reported to co-occur with tunellite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- SrB₆O₁₀·4H₂O
- Mohs hardness
- 2.5
- Density
- 2.40 g/cm³
- Colors
- Streak
- White
- Luster
- Subvitreous, pearly on {100} faces and cleavages
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Nodular, prismatic, tabular
- Cleavage
- On {100}, perfect; on {001}, distinct
- Fluorescence
- Butter yellow under SW and LW UV
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A rare secondary mineral formed in borate deposits
Where rockhounds find tunellite
Classic worldwide localities
- Boron District, California, USA
- Tick Canyon, California, USA
Field-hunting tip
Where it forms: A rare secondary mineral formed in borate deposits. If you start seeing inderite, kurnakovite, ulexite in float, you are in the right ground. Typical habit: nodular, prismatic, tabular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify tunellite?+
Where is tunellite found?+
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Find tunellite on the map
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