Thermessaite is a rare potassium aluminum fluoride mineral typically found in volcanic fumaroles. It most commonly occurs as small, colorless, tabular crystals associated with other halide minerals in high-temperature environments.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Thermessaite

Hardness: 2
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this thermessaite?

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 thermessaite with a known reference. Thermessaite 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. Thermessaite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Thermessaite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: colorless, white.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: tabular crystals, crusts.

Often confused with

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

Often found alongside thermessaite

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

All properties

Chemical formula: K₂[AlF₅]
Mohs hardness: 2
Density: 2.81 g/cm³
Colors: Colorless White
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Orthorhombic
Crystal habit: Tabular Crystals, Crusts
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Fumarolic Deposits
Typical price: $50-300 per specimen

Where rockhounds find thermessaite

Classic worldwide localities

Mount Vesuvius, Italy
Vulcano, Aeolian Islands, Italy

Field-hunting tip

Look in fumarolic deposits country — that is the host setting where thermessaite typically forms. If you start seeing hieratite, malladrite, gypsum in float, you are in the right ground. Field specimens usually show a tabular crystals, crusts habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify thermessaite?+

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

Where is thermessaite found?+

Notable localities include Mount Vesuvius, Italy; Vulcano, Aeolian Islands, Italy.

How much is thermessaite worth?+

Typical asking prices fall in the range of $50-300 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

Is thermessaite safe to handle?+

It contains toxic constituents. Contains fluorine. Wash hands after handling and avoid ingestion or inhalation of dust. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like thermessaite?+

Thermessaite 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 thermessaite?+

Thermessaite commonly co-occurs with Hieratite, Malladrite, Gypsum, Realgar. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does thermessaite form in?+

Thermessaite typically forms in fumarolic deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is thermessaite used for?+

Thermessaite is used in collector.

Find thermessaite on the map

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