Thomsonite-Ca is best known for its attractive, radial, multicolored fan or pea-like patterns found within basalt amygdules. Collectors prize it for its unique fibrous structure that displays beautiful chatoyancy or banding when polished by lapidaries. It is most frequently found in volcanic rocks, particularly within Lake Superior amygdaloids.
Is this thomsonite-ca?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch thomsonite-ca with a known reference. Thomsonite-Ca sits at Mohs 5-5.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to pearly.
- 4Match the color rangeCompare against the expected color range: white, colorless, greenish, yellow, pink.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous, radial, spherical.
Often confused with
Thomsonite-Ca vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside thomsonite-ca
Minerals reported to co-occur with thomsonite-ca. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaCa₂Al₅Si₅O₂₀·6H₂O
- Mohs hardness
- 5-5.5
- Density
- 2.23-2.39 g/cm³
- Streak
- White
- Luster
- Vitreous to pearly
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous, radial, spherical
- Cleavage
- Perfect on {010}, good on {100}
- Rarity
- Uncommon
- Uses
- Collector, Lapidary, Decorative
- Host rock
- In amygdules and fractures in mafic igneous rocks, typically basalts; in some alkalic igneous rocks, contact metamorphic zones, and hypabyssal rocks
Where rockhounds find thomsonite-ca
Classic worldwide localities
- Paterson, Passaic Co., New Jersey, USA
- North and South Table Mountain, Jefferson Co., Colorado, USA
- Springfield, Lane Co., Drain, Douglas Co., Goble, Columbia Co., Ritter Hot Springs, Grant Co., Oregon, USA
- Old Kilpatrick, Dumbartonshire, and Bishopton, Renfrew, Scotland
- Mt. Monzoni, Val di Fassa, Trentino-Alto Adige, Italy
- Ostroh (Seeberg), Kadan (Kaaden), and Ústí nad Lábem (Aussig), Czech Republic
Field-hunting tip
Where it forms: In amygdules and fractures in mafic igneous rocks, typically basalts; in some alkalic igneous rocks, contact metamorphic zones, and hypabyssal rocks. If you start seeing zeolites, calcite, prehnite in float, you are in the right ground. Typical habit: fibrous, radial, spherical. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify thomsonite-ca?+
Where is thomsonite-ca found?+
What rocks look like thomsonite-ca?+
What minerals are found with thomsonite-ca?+
What kind of rock does thomsonite-ca form in?+
What is thomsonite-ca used for?+
Find thomsonite-ca on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
