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. Tobermorite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Tobermorite typically shows a pearly luster.
- 4Match the color rangeCompare against the expected color range: white, pink, yellowish, colorless.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: platy crystals, fibrous, massive, or as coating.
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 pearly on Tobermorite and vitreous to silky on Pectolite.
How to tell apart: Prehnite is the harder of the two (Mohs 6-6.5 vs. 2.5); luster reads pearly on Tobermorite and vitreous on Prehnite.
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
- 2.4-2.5 g/cm³
- Streak
- White
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy Crystals, Fibrous, Massive, Or as Coating
- Cleavage
- Perfect Basal
- Rarity
- Uncommon
- Uses
- Collector, Scientific Research
- Host rock
- Metamorphic Rocks, Hydrothermal Veins in Basalt, And Thermally Metamorphosed Limestone
- Typical price
- $10-60 for small mineral specimens
Where rockhounds find tobermorite
Classic worldwide localities
- Tobermory, Scotland
- Giurudden, Sweden
- Crestmore, California, USA
- Mount Vesuvius, Italy
Field-hunting tip
Look in metamorphic rocks, hydrothermal veins in basalt, and thermally metamorphosed limestone country, the host setting where tobermorite typically forms. If you start seeing calcite, ettringite, portlandite in float, you are in the right ground. Field specimens usually show a platy crystals, fibrous, massive, or as coating habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find tobermorite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
