Fayalite is the iron-rich end member of the olivine solid solution series and is typically found in high-silica volcanic rocks and certain granitic environments. It is noted for its high density and tendency to oxidize into hematite or magnetite when exposed to air. Collectors look for dark, well-formed crystals often embedded in light-colored volcanic matrix.
Is this fayalite?
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 fayalite with a known reference. Fayalite sits at Mohs 6.5-7, 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 to resinous on fractures.
- 4Match the color rangeCompare against the expected color range: yellow, yellow-green, brown, black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: granular, massive.
Often confused with
Fayalite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside fayalite
Minerals reported to co-occur with fayalite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe₂SiO₄
- Mohs hardness
- 6.5-7
- Density
- 4.392 g/cm³
- Streak
- White
- Luster
- Vitreous to resinous on fractures
- Transparency
- Transparent
- Crystal system
- Orthorhombic
- Crystal habit
- Granular, massive
- Cleavage
- {010}, {100}, imperfect
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- In ultramafic volcanic and plutonic rocks, less commonly in felsic plutonic rocks; rarely in granite pegmatite; in lithophysae in obsidian
Where rockhounds find fayalite
Classic worldwide localities
- St. Peters Dome area, near Pikes Peak, El Paso Co., Colorado, USA
- Obsidian Cli®, Yellowstone National Park, Wyoming, USA
- Coso Hot Springs, Inyo Co., California, USA
- Rockport, Essex Co., Massachusetts, USA
- Blue Bell mine, Riondel, British Columbia, Canada
- Brocken massif, Harz Mountains, Germany
Field-hunting tip
Where it forms: In ultramafic volcanic and plutonic rocks, less commonly in felsic plutonic rocks; rarely in granite pegmatite; in lithophysae in obsidian. If you start seeing augite, plagioclase, microcline in float, you are in the right ground. Typical habit: granular, massive. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify fayalite?+
Where is fayalite found?+
What rocks look like fayalite?+
What minerals are found with fayalite?+
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What is fayalite used for?+
Find fayalite on the map
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