Hydromagnesite is a secondary mineral commonly found as crusts or delicate acicular sprays in altered serpentinite deposits. Collectors value it for its frequent and often spectacular blue fluorescence under ultraviolet light.
Is this hydromagnesite?
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 hydromagnesite with a known reference. Hydromagnesite 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. Hydromagnesite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Hydromagnesite typically shows a pearly luster.
- 4Match the color rangeCompare against the expected color range: white, colorless, grayish.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular crystals, radiating clusters, crusts, botryoidal.
Often confused with
Hydromagnesite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Hydromagnesite is noticeably harder (Mohs 3.5 vs. 1-2); luster reads pearly on Hydromagnesite and dull on Huntite.
How to tell apart: Luster reads pearly on Hydromagnesite and vitreous on Magnesite.
How to tell apart: Hydromagnesite is noticeably harder (Mohs 3.5 vs. 2.5); luster reads pearly on Hydromagnesite and vitreous on Artinite.
Often found alongside hydromagnesite
Minerals reported to co-occur with hydromagnesite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₅(CO₃)₄(OH)₂·4H₂O
- Mohs hardness
- 3.5
- Density
- 2.16-2.26 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular Crystals, Radiating Clusters, Crusts, Botryoidal
- Cleavage
- Perfect On {010}
- Fluorescence
- Often Bright Blue or White Under SW/LW UV
- Rarity
- Common
- Uses
- Collector, Scientific Research
- Host rock
- Serpentinite, Altered Ultramafic Rocks, Hydrothermal Veins
- Typical price
- $10-60 per specimen
Where rockhounds find hydromagnesite
Classic worldwide localities
- USA (Pennsylvania, Nevada)
- Italy
- Greece
- Canada (British Columbia)
- Turkey
Field-hunting tip
Look in serpentinite, altered ultramafic rocks, hydrothermal veins country — that is the host setting where hydromagnesite typically forms. If you start seeing magnesite, brucite, calcite in float, you are in the right ground. Field specimens usually show a acicular crystals, radiating clusters, crusts, botryoidal habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find hydromagnesite on the map
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