Thornasite is a rare sodium thorium silicate, typically colorless to very pale green. It was approved as a distinct mineral species in 1985. It was first described from Mont Saint-Hilaire and near Saint-Amable, Quebec, Canada.

Hardness

Mohs

Luster

Vitreous to waxy or greasy, may be pearly

Streak

White

Transparency

Opaque to transparent

mineralrareTerskite Group family

Thornasite

Hardness: 5
Luster: Vitreous to waxy or greasy, may be pearly
Streak: White
Transparency: Opaque to transparent

Is this thornasite?

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 thornasite with a known reference. Thornasite sits at Mohs 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 to waxy or greasy, may be pearly.
4
Match the color range
Compare against the expected color range: colorless, pale green.
5
Look at form & habit
Crystal system: trigonal. Typical habit: anhedral grains, metamict, microscopic crystals.

Often confused with

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

Often found alongside thornasite

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

All properties

Chemical formula: Na₁₂Th₃(Si₈O₁₉)₄·18H₂O
Mohs hardness: 5
Density: 2.62 g/cm³
Colors: Colorless Pale green
Streak: White
Luster: Vitreous to waxy or greasy, may be pearly
Transparency: Opaque to transparent
Crystal system: Trigonal
Crystal habit: Anhedral grains, metamict, microscopic crystals
Cleavage: None
Fluorescence: Bright green under SW and LW UV
Rarity: Rare
Uses: Collector
Host rock: In an intrusive alkalic gabbro-syenite complex (Mont Saint-Hilaire, Canada); in a cavity in a nepheline syenite sill (Saint-Amable, Canada)

Where rockhounds find thornasite

Classic worldwide localities

Mont Saint-Hilaire and near Saint-Amable, Quebec, Canada

Field-hunting tip

Where it forms: In an intrusive alkalic gabbro-syenite complex (Mont Saint-Hilaire, Canada); in a cavity in a nepheline syenite sill (Saint-Amable, Canada). If you start seeing brockite, yofortierite, analcime in float, you are in the right ground. Typical habit: anhedral grains, metamict, microscopic crystals. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify thornasite?+

Mohs hardness is 5. Luster: vitreous to waxy or greasy, may be pearly. The streak is white. Common colors include colorless, pale green.

Where is thornasite found?+

Notable localities include Mont Saint-Hilaire and near Saint-Amable, Quebec, Canada.

Is thornasite safe to handle?+

Thornasite contains uranium or thorium and calls for real care. Contains thorium and is radioactive; store it apart from other specimens. Wash hands thoroughly after handling and avoid creating dust.

What rocks look like thornasite?+

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

What minerals are found with thornasite?+

Thornasite commonly co-occurs with Brockite, Yofortierite, Analcime, Eudialyte, Sérandite, Polylithionite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does thornasite form in?+

Where it forms: In an intrusive alkalic gabbro-syenite complex (Mont Saint-Hilaire, Canada); in a cavity in a nepheline syenite sill (Saint-Amable, Canada). Working float back to the host body is the standard way to chase a fresh occurrence.

What is thornasite used for?+

Thornasite is used in collector.

Find thornasite on the map

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