Rankinite is a rare calcium silicate mineral typically formed during the high-temperature metamorphism of limestone near igneous intrusions. It is usually found as massive or granular aggregates and is highly prized by collectors of contact metamorphic minerals for its scarcity.
Is this rankinite?
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 rankinite with a known reference. Rankinite sits at Mohs 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.
- 4Match the color rangeCompare against the expected color range: colorless, white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: granular, massive, or as rare prismatic crystals.
Often confused with
Rankinite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside rankinite
Minerals reported to co-occur with rankinite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₃Si₂O₇
- Mohs hardness
- 5.5
- Density
- 2.96-3.00 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Granular, massive, or as rare prismatic crystals
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In high-temperature calc-silicate skarns
Where rockhounds find rankinite
Classic worldwide localities
- Christmas Mountains, Brewster Co., Texas, USA
- Scawt Hill, near Larne, Co. Antrim, Ireland
- Golden Gully, Tokatoka district, about 150 km north of Auckland, New Zealand
- Fuka, near Bicchu, Okayama Prefecture, Japan
- Hatrurim Formation, at Hatrurim and Ma'aleh Adumim, Israel
- Lower Tunguska River basin, central Siberia, Russia
Field-hunting tip
Where it forms: In high-temperature calc-silicate skarns. If you start seeing larnite, melilite, spurrite in float, you are in the right ground. Typical habit: granular, massive, or as rare prismatic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify rankinite?+
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What rocks look like rankinite?+
What minerals are found with rankinite?+
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Find rankinite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
