Pigotite is a rare, poorly defined organic mineraloid occurring as brownish, alumina-rich deposits found in granite caves in Cornwall. It is essentially an organic material derived from the weathering of aluminum-bearing minerals by organic acids from vegetation.
Is this pigotite?
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 pigotite with a known reference. Pigotite sits at Mohs 1-2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: yellowish-brown.
- 3Read the lusterHold the specimen under a strong light. Luster: dull.
- 4Match the color rangeCompare against the expected color range: brown, yellowish-brown.
- 5Look at form & habitCrystal system: amorphous. Typical habit: crusts, films, or stalactitic-like aggregates.
Often confused with
Pigotite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Limonite is the harder of the two (Mohs 4-5.5 vs. 1-2); luster reads dull on Pigotite and submetallic to earthy on Limonite.
How to tell apart: Goethite is the harder of the two (Mohs 5-5.5 vs. 1-2); streak differs, so Pigotite leaves yellowish-brown, Goethite leaves brownish yellow, yellow-orange, ocher-yellow; luster reads dull on Pigotite and imperfect adamantine metallic to dull earthy; silky when fibrous on Goethite.
Often found alongside pigotite
Minerals reported to co-occur with pigotite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₄C₆H₅O₁₀·13H₂O (?)
- Mohs hardness
- 1-2
- Colors
- Streak
- Yellowish-brown
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Amorphous
- Crystal habit
- Crusts, films, or stalactitic-like aggregates
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- granite cave systems
Where rockhounds find pigotite
Classic worldwide localities
- Cornwall, England
Field-hunting tip
Where it forms: granite cave systems. If you start seeing quartz in float, you are in the right ground. Typical habit: crusts, films, or stalactitic-like aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify pigotite?+
Where is pigotite found?+
What rocks look like pigotite?+
What minerals are found with pigotite?+
What kind of rock does pigotite form in?+
What is pigotite used for?+
Find pigotite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
