Katoite is a rare member of the garnet group that contains significant amounts of structural hydroxyl groups. It is typically found as small, white to colorless dodecahedral crystals in volcanic contact zones and metamorphosed limestone environments.

Hardness

5-6

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent to translucent

mineralrareGarnet Group family

Katoite

Hardness: 5-6
Luster: Vitreous
Streak: White
Transparency: Transparent to translucent

Is this katoite?

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 katoite with a known reference. Katoite sits at Mohs 5-6, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: colorless, milky white.
5
Look at form & habit
Crystal system: cubic. Typical habit: dodecahedral crystals, massive.

Often confused with

Katoite vs. its common look-alikes, and how to tell them apart in the field.

Often found alongside katoite

Minerals reported to co-occur with katoite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: Ca₃Al₂(OH)₁₂
Mohs hardness: 5-6
Density: 2.76 g/cm³
Colors: Colorless Milky white
Streak: White
Luster: Vitreous
Transparency: Transparent to translucent
Crystal system: Cubic
Crystal habit: Dodecahedral crystals, massive
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: A hydrothermal mineral in cavities in a phonolitic lava flow that erupted through an argillaceous marl

Where rockhounds find katoite

Classic worldwide localities

Campomorto quarry, near Montalto di Castro, Lazio, Italy

Field-hunting tip

Where it forms: A hydrothermal mineral in cavities in a phonolitic lava flow that erupted through an argillaceous marl. If you start seeing tobermorite, afwillite, gehlenite in float, you are in the right ground. Typical habit: dodecahedral crystals, massive. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify katoite?+

Mohs hardness is 5-6. Luster: vitreous. The streak is white. Common colors include colorless, milky white.

Where is katoite found?+

Notable localities include Campomorto quarry, near Montalto di Castro, Lazio, Italy.

What rocks look like katoite?+

Katoite is most often confused with Grossular. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with katoite?+

Katoite commonly co-occurs with Tobermorite, Afwillite, Gehlenite, Hydrocalumite, Opal, Portlandite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does katoite form in?+

Where it forms: A hydrothermal mineral in cavities in a phonolitic lava flow that erupted through an argillaceous marl. Working float back to the host body is the standard way to chase a fresh occurrence.

What is katoite used for?+

Katoite is used in collector.

Find katoite on the map

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