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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
2
Mohs
Luster
Vitreous
Streak
Yellowish
Transparency
Translucent

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.

Hieratite mineral
mineral
Hieratite
Mohs 2

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

Compare →

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
YellowYellowish-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 — that is 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.

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