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.

Hardness

Mohs

Luster

Pearly

Streak

White

Transparency

Translucent

mineralrareLayered Double Hydroxide Group family

Hydrocalumite

Hardness: 2
Luster: Pearly
Streak: White
Transparency: Translucent

Is this hydrocalumite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch hydrocalumite with a known reference. Hydrocalumite sits at Mohs 2 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Hydrocalumite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Hydrocalumite typically shows a pearly luster.
4
Match the color range
Compare against the expected color range: white, colorless.
5
Look at form & habit
Crystal 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.

mineral

Friedelite

Mohs 4-5

How to tell apart: Friedelite is the harder of the two (Mohs 4-5 vs. 2); luster reads pearly on Hydrocalumite and vitreous on Friedelite.

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)₁₂Cl₂(H₂O)₆
Mohs hardness: 2
Density: 2.05 g/cm³
Colors: White Colorless
Streak: White
Luster: Pearly
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Platy Crystals, Pseudo-hexagonal Tablets
Cleavage: Perfect Basal
Rarity: Rare
Uses: Collector, Research
Host rock: Metamorphic Contact Zones, Altered Limestone
Typical price: $20-100 for micro specimens

Where rockhounds find hydrocalumite

Classic worldwide localities

Monzoni, Italy
Etna, Italy
Khibiny Massif, Russia
Neumark, Germany

Field-hunting tip

Look in metamorphic contact zones, altered limestone country — that is the host setting where hydrocalumite typically forms. If you start seeing ettringite, calcite, portlandite in float, you are in the right ground. Field specimens usually show a platy crystals, pseudo-hexagonal tablets habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify hydrocalumite?+

Mohs hardness is 2. It typically shows a pearly luster. The streak is white. Common colors include white, colorless.

Where is hydrocalumite found?+

Notable localities include Monzoni, Italy; Etna, Italy; Khibiny Massif, Russia; Neumark, Germany.

How much is hydrocalumite worth?+

Typical asking prices fall in the range of $20-100 for micro specimens. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like hydrocalumite?+

Hydrocalumite is most often confused with Friedelite, Portlandite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with hydrocalumite?+

Hydrocalumite commonly co-occurs with Ettringite, Calcite, Portlandite, Grossular. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does hydrocalumite form in?+

Hydrocalumite typically forms in metamorphic contact zones, altered limestone. Working float back to the host body is the standard way to chase a fresh occurrence.

What is hydrocalumite used for?+

Hydrocalumite is used in collector, research.

Find hydrocalumite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.