Gerstleyite is an extremely rare sulfosalt mineral found primarily in borate deposits in California. It typically occurs as reddish-orange crusts or small fibrous aggregates associated with colemanite and realgar.
Is this gerstleyite?
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 gerstleyite with a known reference. Gerstleyite 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: bright cinnabar-red; powder darkens on exposure.
- 3Read the lusterHold the specimen under a strong light. Luster: weakly adamantine.
- 4Match the color rangeCompare against the expected color range: vermilion, black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: fibrous, platy, spherical.
Often confused with
Gerstleyite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Gerstleyite leaves bright cinnabar-red; powder darkens on exposure, Realgar leaves red-orange to red; luster reads weakly adamantine on Gerstleyite and resinous to greasy on Realgar.
How to tell apart: Streak differs, so Gerstleyite leaves bright cinnabar-red; powder darkens on exposure, Cinnabar leaves scarlet; luster reads weakly adamantine on Gerstleyite and adamantine, inclining to metallic when dark; dull in friable material on Cinnabar.
Often found alongside gerstleyite
Minerals reported to co-occur with gerstleyite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Na₂(Sb,As)₈S₁₃·2H₂O
- Mohs hardness
- 2.5
- Density
- 3.62 g/cm³
- Colors
- VermilionBlack
- Streak
- Bright cinnabar-red; powder darkens on exposure
- Luster
- Weakly adamantine
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous, platy, spherical
- Cleavage
- Perfect on {010} and {100}; poor on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A low-temperature mineral found embedded in massive borates and clay
Where rockhounds find gerstleyite
Classic worldwide localities
- Baker mine, Kramer borate deposit, Boron, Kern Co., California (type locality), USA
Field-hunting tip
Where it forms: A low-temperature mineral found embedded in massive borates and clay. If you start seeing borax, probertite, tincalconite in float, you are in the right ground. Typical habit: fibrous, platy, spherical. Train your eye for that shape before scanning the outcrop.
Common questions
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Find gerstleyite on the map
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