Demartinite is an extremely rare potassium fluorosilicate mineral found primarily as a fumarolic sublimation product at Mount Vesuvius. It typically forms small, yellow, tabular crystals or delicate crusts in volcanic vents where fluorine-rich gases have interacted with the local environment.

Hardness

Mohs

Luster

Vitreous

Streak

Yellowish

Transparency

Translucent

mineralrare

Demartinite

Hardness: 2
Luster: Vitreous
Streak: Yellowish
Transparency: Translucent

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Is this demartinite?

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

Often confused with

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

mineral

Hieratite

Mohs 2

How to tell apart: Streak differs, so Demartinite leaves yellowish, Hieratite leaves white.

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Often found alongside demartinite

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

All properties

Chemical formula: K₂SiF₆
Mohs hardness: 2
Density: 2.65 g/cm³
Colors: Yellow Yellowish-green
Streak: Yellowish
Luster: Vitreous
Transparency: Translucent
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 demartinite

Classic worldwide localities

Vesuvius (Italy)

Field-hunting tip

Look in fumarolic deposits country, the host setting where demartinite typically forms. If you start seeing malladrite, avogadrite, salmiac 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 demartinite?+

Mohs hardness is 2. It typically shows a vitreous luster. The streak is yellowish. Common colors include yellow, yellowish-green.

Where is demartinite found?+

Notable localities include Vesuvius (Italy).

How much is demartinite 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 demartinite safe to handle?+

It contains toxic constituents. Contains fluorine and potassium salts; avoid ingestion or inhalation of dust. Wear gloves and wash hands after handling. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like demartinite?+

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

What minerals are found with demartinite?+

Demartinite commonly co-occurs with Malladrite, Avogadrite, Salmiac. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does demartinite form in?+

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

What is demartinite used for?+

Demartinite is used in collector.

Find demartinite on the map

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