Prosopite is a rare calcium aluminum fluoride mineral typically found as a secondary alteration product in tin-bearing greisens. It most commonly appears as dull, massive, or granular masses rather than well-defined crystals, often forming as a pseudomorph after minerals like topaz or fluorite.
Is this prosopite?
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 prosopite with a known reference. Prosopite sits at Mohs 4.5, 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: vitreous, weak.
- 4Match the color rangeCompare against the expected color range: white, greenish white, reddish white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: crystalline, massive.
Often confused with
Prosopite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside prosopite
Minerals reported to co-occur with prosopite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaAl₂(F,OH)₈
- Mohs hardness
- 4.5
- Density
- 2.880-2.894 g/cm³
- Streak
- White
- Luster
- Vitreous, weak
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Crystalline, massive
- Cleavage
- {111}, perfect
- Fluorescence
- Sky blue blue white under SW and LW UV
- Rarity
- Uncommon
- Uses
- Collector
- Host rock
- Disseminated in fluorine-rich granites, greisens, and granite pegmatites; more rarely in base-metal deposits
Where rockhounds find prosopite
Classic worldwide localities
- St. Peters Dome, near Pikes Peak, El Paso Co., and in the Goldie carbonatite, McClure Mountain-Iron Mountain, Fremont Co., Colorado, USA
- Dugway district, Tooele Co., Utah, USA
- Zapot pegmatite, 25 km northeast of Hawthorne, Fitting district, Mineral Co., Nevada, USA
- Grand Reef mine, Aravaipa district, Graham Co., and 16 km north of Bouse, La Paz Co., Arizona, USA
- Altenberg, Saxony, Germany
- Horní Slavkov (Schlaggenwald), Czech Republic
Field-hunting tip
Where it forms: Disseminated in fluorine-rich granites, greisens, and granite pegmatites; more rarely in base-metal deposits. If you start seeing fluorite, cryolite, thomsenolite in float, you are in the right ground. Typical habit: crystalline, massive. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify prosopite?+
Where is prosopite found?+
What rocks look like prosopite?+
What minerals are found with prosopite?+
What kind of rock does prosopite form in?+
What is prosopite used for?+
Find prosopite on the map
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