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. Mcguinnessite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Mcguinnessite typically shows a pearly luster.
- 4Match the color rangeCompare against the expected color range: blue, greenish-blue, light blue.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: botryoidal, crusts, radial aggregates.
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 — Mcguinnessite leaves white, Malachite leaves light green; luster reads pearly on Mcguinnessite and vitreous on Malachite.
How to tell apart: Streak differs — Mcguinnessite leaves white, Aurichalcite leaves pale blue.
How to tell apart: Luster reads pearly on Mcguinnessite and dull 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
- 3.16 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Botryoidal, Crusts, Radial Aggregates
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Serpentinite
- Typical price
- $20-150 for micro-mounts or small cabinet specimens
Where rockhounds find mcguinnessite
Classic worldwide localities
- Red Mountain, Mendocino County, California, USA
- Santa Lucia Mountains, California, USA
- Kamativi Mine, Zimbabwe
- Zajacice, Czech Republic
Field-hunting tip
Look in serpentinite country — that is the host setting where mcguinnessite typically forms. If you start seeing magnesite, hydromagnesite, serpentine in float, you are in the right ground. Field specimens usually show a botryoidal, crusts, radial aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find mcguinnessite on the map
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