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. Penberthycroftite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Penberthycroftite typically shows a vitreous to pearly luster.
- 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 to adamantine 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
- Hydrothermal Veins in Oxidized Base Metal Deposits
Where rockhounds find penberthycroftite
Classic worldwide localities
- Penberthy Croft Mine, Cornwall, England
- Tsumeb Mine, Namibia
Field-hunting tip
Look in hydrothermal veins in oxidized base metal deposits country, the host setting where penberthycroftite typically forms. If you start seeing goethite, mimetite, cornwallite in float, you are in the right ground. Field specimens usually show a crusts of rectangular laths habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find penberthycroftite on the map
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