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 1.5-2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Gerstleyite leaves a bright cinnabar-red streak.
- 3Read the lusterHold the specimen under a strong light. Gerstleyite typically shows a adamantine luster.
- 4Match the color rangeCompare against the expected color range: red, orange, brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: massive, crusts, or radiating fibrous aggregates.
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, Realgar leaves orange-red; luster reads adamantine on Gerstleyite and resinous on Realgar.
How to tell apart: Streak differs, so Gerstleyite leaves bright cinnabar-red, Cinnabar leaves scarlet.
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
- 1.5-2
- Density
- 3.5 g/cm³
- Streak
- Bright Cinnabar-red
- Luster
- Adamantine
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Massive, Crusts, Or Radiating Fibrous Aggregates
- Cleavage
- Perfect in One Direction
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Evaporite Deposits in Borate Mines
Where rockhounds find gerstleyite
Classic worldwide localities
- Boron, California, USA
Field-hunting tip
Look in evaporite deposits in borate mines country, the host setting where gerstleyite typically forms. If you start seeing colemanite, ulexite, realgar in float, you are in the right ground. Field specimens usually show a massive, crusts, or radiating fibrous aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify gerstleyite?+
Where is gerstleyite found?+
Is gerstleyite safe to handle?+
What rocks look like gerstleyite?+
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Find gerstleyite on the map
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