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, 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.