Penberthycroftite is a rare aluminum arsenate that forms white crusts of rectangular laths. It is exceptionally rare, occurring in the oxidized zone of a multi-stage, polymetallic hydrothermal deposit. It was first described from the Penberthy Croft mine, St. Hilary, Cornwall, England.
Is this penberthycroftite?
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 penberthycroftite with a known reference. Penberthycroftite sits at Mohs 3.5, 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 to pearly.
- 4Match the color rangeCompare against the expected color range: white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: crusts of rectangular laths.
Often confused with
Penberthycroftite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Penberthycroftite leaves white, Descloizite leaves orange to brownish red; luster reads vitreous to pearly on Penberthycroftite and greasy on Descloizite.
How to tell apart: Luster reads vitreous to pearly on Penberthycroftite and resinous to subadamantine on Mimetite.
Often found alongside penberthycroftite
Minerals reported to co-occur with penberthycroftite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- [Al₆(AsO₄)₃(OH)₉(H₂O)₅]·8H₂O
- Mohs hardness
- 3.5
- Density
- 2.18 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous to pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Crusts of rectangular laths
- Cleavage
- Perfect on {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Exceptionally rare, in the oxidized zone of a multi-stage, polymetallic hydrothermal deposit
Where rockhounds find penberthycroftite
Classic worldwide localities
- Penberthy Croft mine, St. Hilary, Cornwall, England
Field-hunting tip
Where it forms: Exceptionally rare, in the oxidized zone of a multi-stage, polymetallic hydrothermal deposit. If you start seeing arsenopyrite, bettertonite, bulachite in float, you are in the right ground. Typical habit: crusts of rectangular laths. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify penberthycroftite?+
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Find penberthycroftite on the map
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