Cryptohalite is a rare ammonium fluorosilicate typically found as a sublimation product around volcanic fumaroles. It occurs as small, delicate hexagonal crystals or thin crusts, often in association with other fumarolic minerals like sulfur and sal ammoniac.

Hardness

2.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Cryptohalite

Hardness: 2.5
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this cryptohalite?

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 cryptohalite with a known reference. Cryptohalite sits at Mohs 2.5, 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, gray, white.
5
Look at form & habit
Crystal system: cubic. Typical habit: crusts of interpenetrating tabular crystals.

Often confused with

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

Often found alongside cryptohalite

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

All properties

Chemical formula: (NH₄)₂SiF₆
Mohs hardness: 2.5
Density: 2.004 g/cm³
Colors: Colorless Gray White
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Cubic
Crystal habit: Crusts of interpenetrating tabular crystals
Cleavage: Perfect on {111}
Rarity: Rare
Uses: Collector
Host rock: A volcanic sublimate, or formed from fires in burning coal seams

Where rockhounds find cryptohalite

Classic worldwide localities

Kehley’s Run mine, near Shenandoah, Schuylkill Co., and at Glen Lyon, Luzerne Co., Pennsylvania, USA
Vesuvius, Campania, Italy
Clara mine, near Wolfach, Black Forest, Germany
Libusín, Kladno basin, Czech Republic
Bararee mine, Barari, Jharia district, West Bengal, India

Field-hunting tip

Where it forms: A volcanic sublimate, or formed from fires in burning coal seams. If you start seeing sal ammoniac, bararite, sulfur in float, you are in the right ground. Typical habit: crusts of interpenetrating tabular crystals. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify cryptohalite?+

Mohs hardness is 2.5. Luster: vitreous. The streak is white. Common colors include colorless, gray, white.

Where is cryptohalite found?+

Notable localities include Kehley’s Run mine, near Shenandoah, Schuylkill Co., and at Glen Lyon, Luzerne Co., Pennsylvania, USA; Vesuvius, Campania, Italy; Clara mine, near Wolfach, Black Forest, Germany; Libusín, Kladno basin, Czech Republic; Bararee mine, Barari, Jharia district, West Bengal, India.

Is cryptohalite safe to handle?+

Cryptohalite contains soluble fluoride. Contains fluorine and ammonium; toxic if ingested or inhaled as dust. Wash hands thoroughly after handling and avoid dust generation.

What rocks look like cryptohalite?+

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

What minerals are found with cryptohalite?+

Cryptohalite commonly co-occurs with Sal ammoniac, Bararite, Sulfur, Selenium. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does cryptohalite form in?+

Where it forms: A volcanic sublimate, or formed from fires in burning coal seams. Working float back to the host body is the standard way to chase a fresh occurrence.

What is cryptohalite used for?+

Cryptohalite is used in collector.

Find cryptohalite on the map

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