Coalingite is a rare magnesium-iron carbonate hydroxide mineral typically found as small, golden-yellow to orange-brown plates within serpentinized rocks. Collectors primarily find it as thin coatings or small, micaceous patches in association with other serpentine minerals and brucite. It is most famous for its occurrences in the New Idria district of California.
Is this coalingite?
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 coalingite with a known reference. Coalingite 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: yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: resinous.
- 4Match the color rangeCompare against the expected color range: bronze brown, light brown, reddish brown.
- 5Look at form & habitCrystal system: trigonal. Typical habit: platy, micaceous, massive, or as veinlets.
Often confused with
Coalingite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Coalingite leaves yellow, Pyroaurite leaves white; luster reads resinous on Coalingite and waxy to vitreous, may be pearly on Pyroaurite.
How to tell apart: Streak differs, so Coalingite leaves yellow, Hydrotalcite leaves white; luster reads resinous on Coalingite and satiny to greasy or waxy on Hydrotalcite.
How to tell apart: Streak differs, so Coalingite leaves yellow, Brucite leaves white; luster reads resinous on Coalingite and waxy, pearly on cleavage surfaces on Brucite.
Often found alongside coalingite
Minerals reported to co-occur with coalingite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₁₀Fe₂CO₃(OH)₂₄·2H₂O
- Mohs hardness
- 1-2
- Density
- 2.33-2.42 g/cm³
- Streak
- Yellow
- Luster
- Resinous
- Transparency
- Translucent
- Crystal system
- Trigonal
- Crystal habit
- Platy, micaceous, massive, or as veinlets
- Cleavage
- {0001}, perfect; another at 45°
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Developed from iron-rich brucite in the surface weathering zone of a serpentinite (near Coalinga, California, USA)
Where rockhounds find coalingite
Classic worldwide localities
- Rockville quarry, Rockville, Montgomery Co., Maryland, USA
- Cedar Hill quarry, Lancaster Co., Pennsylvania, USA
- Muskox intrusion, Coppermine River area, Northwest Territories, Canada
- Hagdale, Unst, Shetland Islands, Scotland
- Montjovet, Piedmont, Italy
- Lower Tagil massif, Ural Mountains, Russia
Field-hunting tip
Where it forms: Developed from iron-rich brucite in the surface weathering zone of a serpentinite (near Coalinga, California, USA). If you start seeing lizardite, antigorite, chrysotile in float, you are in the right ground. Typical habit: platy, micaceous, massive, or as veinlets. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify coalingite?+
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What rocks look like coalingite?+
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Find coalingite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
