Hydrocalumite is a rare layered double hydroxide mineral typically found in metamorphosed limestone or volcanic ejecta. It is known for its thin, pseudo-hexagonal platy crystals and distinct pearly luster on cleavage surfaces.
Is this hydrocalumite?
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 hydrocalumite with a known reference. Hydrocalumite sits at Mohs 3, 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, pearly on cleavages.
- 4Match the color rangeCompare against the expected color range: colorless, light green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy crystals, pseudo-hexagonal tablets.
Often confused with
Hydrocalumite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Friedelite is the harder of the two (Mohs 4-5 vs. 3); streak differs, so Hydrocalumite leaves white, Friedelite leaves pale rose.
How to tell apart: Hydrocalumite is noticeably harder (Mohs 3 vs. 2); luster reads vitreous, pearly on cleavages on Hydrocalumite and pearly on cleavages on Portlandite.
Often found alongside hydrocalumite
Minerals reported to co-occur with hydrocalumite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₄Al₂(OH)₁₂(Cl,CO₃,OH)₂·4H₂O
- Mohs hardness
- 3
- Density
- 2.09-2.15 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous, pearly on cleavages
- Transparency
- Transparent
- Crystal system
- Monoclinic
- Crystal habit
- Platy crystals, pseudo-hexagonal tablets
- Cleavage
- {001}, perfect; another inclined at ∼60° to {100}, poor
- Rarity
- Rare
- Uses
- Collector, Research
- Host rock
- A rare late-stage hydrothermal mineral in skarns formed from contact metamorphism of limestone or in xenoliths in lava
Where rockhounds find hydrocalumite
Classic worldwide localities
- Scawt Hill, near Larne, Co. Antrim, Ireland
- Boisséjour, near Ceyrat, Puy-de-Dôme, France
- Campomorto, Montalto di Castro, Lazio, Italy
- Bellerberg, Rothenberg, and Emmelberg volcanos, near Mayen, Eifel district, Germany
- Kopeysk, Chelyabinsk coal basin, Southern Ural Mountains, Russia
- Daba marbles, Jordan
Field-hunting tip
Where it forms: A rare late-stage hydrothermal mineral in skarns formed from contact metamorphism of limestone or in xenoliths in lava. If you start seeing afwillite, portlandite, ettringite in float, you are in the right ground. Typical habit: platy crystals, pseudo-hexagonal tablets. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify hydrocalumite?+
Where is hydrocalumite found?+
What rocks look like hydrocalumite?+
What minerals are found with hydrocalumite?+
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Find hydrocalumite on the map
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