Mcguinnessite is a rare copper-magnesium carbonate that typically forms delicate, pale blue, botryoidal or radial crusts on serpentine host rocks. It is highly sought after by micromount collectors due to its distinctive color and association with nickel-rich serpentine environments.
Is this mcguinnessite?
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 mcguinnessite with a known reference. Mcguinnessite sits at Mohs 2.5, 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 silky.
- 4Match the color rangeCompare against the expected color range: blue green, green, white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: spherical.
Often confused with
Mcguinnessite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Malachite is the harder of the two (Mohs 3.5-4 vs. 2.5); streak differs, so Mcguinnessite leaves white, Malachite leaves pale green; luster reads vitreous to silky on Mcguinnessite and adamantine to vitreous; silky if fibrous; dull to earthy if massive on Malachite.
How to tell apart: Streak differs, so Mcguinnessite leaves white, Aurichalcite leaves light blue; luster reads vitreous to silky on Mcguinnessite and silky to pearly on Aurichalcite.
How to tell apart: Luster reads vitreous to silky on Mcguinnessite and pearly, silky to dull or earthy in aggregates on Hydrozincite.
Often found alongside mcguinnessite
Minerals reported to co-occur with mcguinnessite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Mg,Cu)₂CO₃(OH)₂
- Mohs hardness
- 2.5
- Density
- 3.02-3.22 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous to silky
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Spherical
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A rare secondary mineral, typically formed in serpentinized peridotite
Where rockhounds find mcguinnessite
Classic worldwide localities
- Red Mountain, 24 km southeast of Ukiah, Mendocino Co., California, USA
- Occurs in the Rockville quarry, six km west of Rockville, Montgomery Co., Maryland, USA
- Cedar Hill quarry, Lancaster Co., Pennsylvania, USA
- Gabbs mine, Gabbs district, Nye Co., and from the Chalk Mountain mine and other claims in the Chalk Mountain district, Churchill Co., Nevada, USA
- Arroyo Puerto Nuevo, near San José de Castro, Baja California, Mexico
- Kraubath serpentine massif, Styria, Austria
Field-hunting tip
Where it forms: A rare secondary mineral, typically formed in serpentinized peridotite. If you start seeing vuagnatite, malachite, azurite in float, you are in the right ground. Typical habit: spherical. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify mcguinnessite?+
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Is mcguinnessite safe to handle?+
What rocks look like mcguinnessite?+
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Find mcguinnessite on the map
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