Tilleyite is a relatively rare calcium silicate-carbonate mineral found in contact metamorphic zones where limestone is altered by igneous intrusions. It typically occurs as white or colorless tabular crystals or in massive aggregates associated with other contact-metamorphic minerals like wollastonite and grossular garnet.
Is this tilleyite?
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 tilleyite with a known reference. Tilleyite sits at Mohs 3 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Tilleyite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Tilleyite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: colorless, white, grayish.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: tabular crystals, massive, granular.
Often confused with
Tilleyite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside tilleyite
Minerals reported to co-occur with tilleyite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₅Si₂O₇(CO₃)₂
- Mohs hardness
- 3
- Density
- 2.8 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Tabular Crystals, Massive, Granular
- Cleavage
- Distinct in One Direction
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Contact Metamorphosed Limestone
- Typical price
- $20-150 per specimen depending on size and quality
Where rockhounds find tilleyite
Classic worldwide localities
- Crestmore, California
- Carlingford, Ireland
- Tokatoka, New Zealand
- Fuka, Japan
Field-hunting tip
Look in contact metamorphosed limestone country — that is the host setting where tilleyite typically forms. If you start seeing calcite, wollastonite, grossular in float, you are in the right ground. Field specimens usually show a tabular crystals, massive, granular habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify tilleyite?+
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Find tilleyite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
