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. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: dull.
- 4Match the color rangeCompare against the expected color range: colorless, white.
- 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.
How to tell apart: Luster reads dull on Tilleyite and vitreous; pearly on cleavages and {0001} on Calcite.
How to tell apart: Wollastonite is the harder of the two (Mohs 4.5-5 vs. 3); luster reads dull on Tilleyite and vitreous, pearly on cleavage on Wollastonite.
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
- 2.82-2.84 g/cm³
- Streak
- White
- Luster
- Dull
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Tabular crystals, massive, granular
- Cleavage
- Perfect on {201}; poor on {100} and {010}; very poor on {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In the contact metamorphic zone between igneous rocks and limestones, formed at low pressure and high temperature
Where rockhounds find tilleyite
Classic worldwide localities
- Crestmore, Riverside Co., California, USA
- North Peak, Iron Mountains, Sierra Co., New Mexico, USA
- Helena, Lewis and Clark Co., Montana, USA
- Barnavave, near Carlingford, Co. Louth, Ireland
- Güneyce-_ Ikizdere area, Trabzon Province, Turkey
- Anakit Creek, at the mouth of the Lower Tunguska River, central Siberia, Russia
Field-hunting tip
Where it forms: In the contact metamorphic zone between igneous rocks and limestones, formed at low pressure and high temperature. If you start seeing merwinite, spurrite, gehlenite in float, you are in the right ground. Typical habit: tabular crystals, massive, granular. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify tilleyite?+
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What rocks look like tilleyite?+
What minerals are found with tilleyite?+
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Find tilleyite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
