Foshagite is a rare calcium silicate mineral typically found as delicate, fibrous, or acicular white masses in contact metamorphic zones. It is most famous for its occurrences in the limestone quarries of Crestmore, California, where it forms in skarn deposits alongside other calcium-rich minerals.
Is this foshagite?
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 foshagite with a known reference. Foshagite 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: silky.
- 4Match the color rangeCompare against the expected color range: snow white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular, fibrous.
Often confused with
Foshagite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Wollastonite is the harder of the two (Mohs 4.5-5 vs. 3); luster reads silky on Foshagite and vitreous, pearly on cleavage on Wollastonite.
How to tell apart: Xonotlite is the harder of the two (Mohs 6-6.5 vs. 3); luster reads silky on Foshagite and pearly, greasy on Xonotlite.
How to tell apart: Pectolite is the harder of the two (Mohs 4.5-5 vs. 3).
Often found alongside foshagite
Minerals reported to co-occur with foshagite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₄Si₃O₉(OH)₂
- Mohs hardness
- 3
- Density
- 2.73 g/cm³
- Colors
- Streak
- White
- Luster
- Silky
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular, fibrous
- Cleavage
- Distinct on {001}
- Fluorescence
- Yellowish to blue white under SW UV; butter yellow under LW UV
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In thin veins in thermally altered limestone (Crestmore, California, USA; Kilchoan, Scotland) or melilite skarn (Dupezeh Mountain, Iraq)
Where rockhounds find foshagite
Classic worldwide localities
- Crestmore, Riverside Co., California, USA
- Kilchoan, Ardnamurchan, Argyllshire, Scotland
- Dupezeh Mountain, near Hero Town, Qala-Diza region, Iraq
- Hatrurim Formation, Israel
- Kushiro, Hiroshima Prefecture, and in the Akagané mine, Iwate Prefecture, Japan
- Wessels mine, near Kuruman, Cape Province, South Africa
Field-hunting tip
Where it forms: In thin veins in thermally altered limestone (Crestmore, California, USA; Kilchoan, Scotland) or melilite skarn (Dupezeh Mountain, Iraq). If you start seeing hillebrandite, calcite, vesuvianite in float, you are in the right ground. Typical habit: acicular, fibrous. Train your eye for that shape before scanning the outcrop.
Common questions
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Find foshagite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
