Mercallite is a rare potassium hydrogen sulfate mineral that typically forms as a white efflorescent crust or coating around active volcanic fumaroles. Due to its high solubility, it is rarely found in nature unless collected immediately from dry, arid volcanic environments.
Is this mercallite?
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 mercallite with a known reference. Mercallite sits at Mohs 2, 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, sky blue.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: stalactitic, tabular.
Often confused with
Mercallite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside mercallite
Minerals reported to co-occur with mercallite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KHSO₄
- Mohs hardness
- 2
- Density
- 2.31 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Orthorhombic
- Crystal habit
- Stalactitic, tabular
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In fumaroles
Where rockhounds find mercallite
Classic worldwide localities
- Vesuvius, Campania, Italy
Field-hunting tip
Where it forms: In fumaroles. If you start seeing halite, misenite, hieratite in float, you are in the right ground. Typical habit: stalactitic, tabular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify mercallite?+
Where is mercallite found?+
What rocks look like mercallite?+
What minerals are found with mercallite?+
What kind of rock does mercallite form in?+
What is mercallite used for?+
Find mercallite on the map
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