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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous; silky to pearly to dull in aggregates.
- 4Match the color rangeCompare against the expected color range: white, colorless.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular, encrustations.
Often confused with
Hydromagnesite vs. its common look-alikes, and how to tell them apart in the field.
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
- 2.24-2.25 g/cm³
- Streak
- White
- Luster
- Vitreous; silky to pearly to dull in aggregates
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular, encrustations
- Cleavage
- {010}, perfect
- Fluorescence
- Green under SW UV; bluish white under LW UV
- Rarity
- Common
- Uses
- Collector, Scientific research
- Host rock
- Incrusting and filling fractures in altered ultramafic rocks and serpentinites; in low-temperature hydrothermally altered dolomitic xenoliths and marble; common in caves as a component of “moonmilk” and speleothems
Where rockhounds find hydromagnesite
Classic worldwide localities
- Hoboken, Hudson Co., New Jersey, USA
- Wood’s Chrome and Low’s mines, near Texas, Lancaster Co., Pennsylvania, USA
- Luning, Mineral Co., and near Gabbs, Gabbs district, Nye Co., Nevada, USA
- Crestmore quarry, Riverside Co., California, USA
- Cariboo, Atlin, and Kamloops districts, British Columbia, Canada
- São Felix do Minato mine, Bahia, Brazil
Field-hunting tip
Where it forms: Incrusting and filling fractures in altered ultramafic rocks and serpentinites; in low-temperature hydrothermally altered dolomitic xenoliths and marble; common in caves as a component of “moonmilk” and speleothems. If you start seeing calcite, dolomite, aragonite in float, you are in the right ground. Typical habit: acicular, encrustations. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify hydromagnesite?+
Where is hydromagnesite found?+
What rocks look like hydromagnesite?+
What minerals are found with hydromagnesite?+
What kind of rock does hydromagnesite form in?+
What is hydromagnesite used for?+
Find hydromagnesite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
