Tobermorite is a calcium silicate hydrate mineral often found as a secondary mineral in altered volcanic rocks or contact-metamorphosed limestone. Collectors typically look for its characteristic pearly, thin, platy or fibrous white to pinkish clusters that form in vugs or cavities.
Is this tobermorite?
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 tobermorite with a known reference. Tobermorite sits at Mohs 2.5, 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, silky in fibrous aggregates.
- 4Match the color rangeCompare against the expected color range: white, light pink white, reddish white.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: aggregates, fibrous, radial.
Often confused with
Tobermorite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Pectolite is the harder of the two (Mohs 4.5-5 vs. 2.5); luster reads vitreous, silky in fibrous aggregates on Tobermorite and silky, subvitreous on Pectolite.
How to tell apart: Prehnite is the harder of the two (Mohs 6-6.5 vs. 2.5).
Often found alongside tobermorite
Minerals reported to co-occur with tobermorite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₅Si₆O₁₆(OH)₂·4H₂O
- Mohs hardness
- 2.5
- Density
- 2.423-2.458 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous, silky in fibrous aggregates
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Aggregates, fibrous, radial
- Cleavage
- Perfect on {001}, imperfect on {100}
- Fluorescence
- Weak white to yellow under SW and LW UV
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- A hydrothermal alteration product of calcium carbonate rocks, due to contact metamorphism and metasomatism; filling vesicles and cavities in basaltic rocks
Where rockhounds find tobermorite
Classic worldwide localities
- Crestmore, Riverside Co., California, USA
- Isle of Mull, around Tobermory, Scotland
- Isle of Skye, at Dunvegan and Portree, Scotland
- Ardtornish Bay, Morvern, Argyllshire, Scotland
- Ballycraigy, near Larne, Co. Antrim, Ireland
- Zeilberg, near Maroldsweisach, Bavaria, Germany
Field-hunting tip
Where it forms: A hydrothermal alteration product of calcium carbonate rocks, due to contact metamorphism and metasomatism; filling vesicles and cavities in basaltic rocks. If you start seeing zeolites, ettringite, portlandite in float, you are in the right ground. Typical habit: aggregates, fibrous, radial. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify tobermorite?+
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What rocks look like tobermorite?+
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Find tobermorite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
