Apjohnite is a secondary sulfate mineral that typically forms as delicate, fibrous efflorescences in mine workings and volcanic environments. Because it is highly soluble in water and dehydration-prone, collectors should store specimens in airtight containers to prevent them from crumbling into powder.
Is this apjohnite?
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 apjohnite with a known reference. Apjohnite sits at Mohs 1.5, 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: silky in aggregates.
- 4Match the color rangeCompare against the expected color range: white, colorless, pale yellow, pale green, pale pink.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: encrustations, fibrous.
Often confused with
Apjohnite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads silky in aggregates on Apjohnite and vitreous on Halotrichite.
How to tell apart: Luster reads silky in aggregates on Apjohnite and vitreous on Pickeringite.
How to tell apart: Luster reads silky in aggregates on Apjohnite and vitreous to silky on Alunogen.
Often found alongside apjohnite
Minerals reported to co-occur with apjohnite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- MnAl₂(SO₄)₄·22H₂O
- Mohs hardness
- 1.5
- Density
- 1.78-1.81 g/cm³
- Streak
- White
- Luster
- Silky in aggregates
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Encrustations, fibrous
- Cleavage
- Distinct on {110}
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- As efflorescences
Where rockhounds find apjohnite
Classic worldwide localities
- Alum Cave, Sevier Co., Tennessee, USA
- Alum Point, Salt Lake Co., Utah, USA
- Creede, Mineral Co., Colorado, USA
- Twentieth Century mine, Nye Co., and long fibers at the Urania mine, Cactus Peak district, Nevada, USA
- Maputo (Delagoa) Bay, near Maputo (Lourenço Marques), Mozambique
- Terlano, Bolzano, Trentino-Alto Adige, Italy
Field-hunting tip
Where it forms: As efflorescences. If you start seeing halotrichite, pickeringite, gunningite in float, you are in the right ground. Typical habit: encrustations, fibrous. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify apjohnite?+
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Find apjohnite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
