Hydrotalcite is a layered double hydroxide mineral typically forming soft, waxy or pearly masses and thin hexagonal plates. It is most commonly identified by its low hardness and association with serpentinite or contact-metamorphosed carbonate rocks. Collectors look for its characteristic micaceous habit and smooth, greasy feel.
Is this hydrotalcite?
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 hydrotalcite with a known reference. Hydrotalcite sits at Mohs 2, 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: satiny to greasy or waxy.
- 4Match the color rangeCompare against the expected color range: white, colorless.
- 5Look at form & habitCrystal system: trigonal. Typical habit: foliated, lamellar, massive.
Often confused with
Hydrotalcite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Hydrotalcite is noticeably harder (Mohs 2 vs. 1); luster reads satiny to greasy or waxy on Hydrotalcite and pearly, greasy, dull on Talc.
How to tell apart: Luster reads satiny to greasy or waxy on Hydrotalcite and waxy, pearly on cleavage surfaces on Brucite.
How to tell apart: Luster reads satiny to greasy or waxy on Hydrotalcite and pearly to dull on Pyrophyllite.
Often found alongside hydrotalcite
Minerals reported to co-occur with hydrotalcite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₆Al₂CO₃(OH)₁₆·4H₂O
- Mohs hardness
- 2
- Density
- 2.03-2.09 g/cm³
- Streak
- White
- Luster
- Satiny to greasy or waxy
- Transparency
- Transparent
- Crystal system
- Trigonal
- Crystal habit
- Foliated, lamellar, massive
- Cleavage
- {0001}, perfect
- Rarity
- Common
- Uses
- Collector, Scientific research
- Host rock
- A secondary mineral in serpentinite
Where rockhounds find hydrotalcite
Classic worldwide localities
- Amity, Orange Co., and Somerville, St. Lawrence Co., New York, USA
- Franklin, Sussex Co., New Jersey, USA
- Snarum, Norway
- Tonezza, Vicenza, Italy
- Věžná, Czech Republic
- Found at Goles, Bosnia-Herzegovina
Field-hunting tip
Where it forms: A secondary mineral in serpentinite. If you start seeing manasseite, serpentine, dolomite in float, you are in the right ground. Typical habit: foliated, lamellar, massive. Train your eye for that shape before scanning the outcrop.
Common questions
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Find hydrotalcite on the map
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