Alumohydrocalcite is a rare hydrous calcium aluminum carbonate typically found as delicate, fibrous, or radiating spherical aggregates in alkalic igneous environments. Collectors prize it for its unique, delicate habit, though it is often small and requires magnification to appreciate its radial crystal structures.
Is this alumohydrocalcite?
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 alumohydrocalcite with a known reference. Alumohydrocalcite 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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to pearly, earthy.
- 4Match the color rangeCompare against the expected color range: chalky white, bluish white, violet, light yellow, gray.
- 5Look at form & habitCrystal system: triclinic. Typical habit: acicular, massive, spherical.
Often confused with
Alumohydrocalcite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads vitreous to pearly, earthy on Alumohydrocalcite and satiny to greasy or waxy on Hydrotalcite.
How to tell apart: Luster reads vitreous to pearly, earthy on Alumohydrocalcite and dull on Huntite.
Often found alongside alumohydrocalcite
Minerals reported to co-occur with alumohydrocalcite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaAl₂(CO₃)₂(OH)₄·3H₂O
- Mohs hardness
- 2.5
- Density
- 2.21-2.24 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous to pearly, earthy
- Transparency
- Transparent to opaque
- Crystal system
- Triclinic
- Crystal habit
- Acicular, massive, spherical
- Cleavage
- {100} perfect; {010} imperfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Formed at low temperature by the action of carbonated waters on allophane or dickite in dolomites and limestones; chromian varieties are typically formed near altering chromite from serpentinites
Where rockhounds find alumohydrocalcite
Classic worldwide localities
- East of Mount Hamilton, Santa Clara Co., California, USA
- A Cu–V orebody near Potekhina village, 40 km northeast of Sorsk, Khakassia district, Siberia, Russia
- A dolomite quarry near Bergisch-Gladbach, from the Kalem quarry, near Birresborn, Eifel, and at Ramsbeck, North Rhine-Westphalia, Germany
- Ladomirov, Slovakia
- Bicske-Csordakút, Hungary
- Scarborough, North Yorkshire, England
Field-hunting tip
Where it forms: Formed at low temperature by the action of carbonated waters on allophane or dickite in dolomites and limestones; chromian varieties are typically formed near altering chromite from serpentinites. If you start seeing allophane, dickite, gibbsite in float, you are in the right ground. Typical habit: acicular, massive, spherical. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify alumohydrocalcite?+
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What rocks look like alumohydrocalcite?+
What minerals are found with alumohydrocalcite?+
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Find alumohydrocalcite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
