Ferrosilite is an iron-rich member of the orthopyroxene group, typically found in high-grade metamorphic rocks and sometimes igneous rocks. It is noted for its dark, often brownish-black color and is chemically the iron end-member of the enstatite-ferrosilite series. Collectors should look for its characteristic cleavage and associated iron-rich mineral suites in metamorphic complexes.
Is this ferrosilite?
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 ferrosilite with a known reference. Ferrosilite sits at Mohs 5-6, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: pale brownish gray.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: colorless, green, dark brown, nearly black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: anhedral grains.
Often confused with
Ferrosilite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Ferrosilite leaves pale brownish gray, Enstatite leaves white to grayish.
How to tell apart: Streak differs, so Ferrosilite leaves pale brownish gray, Hypersthene leaves grayish greenish white.
How to tell apart: Streak differs, so Ferrosilite leaves pale brownish gray, Augite leaves gray-green.
Often found alongside ferrosilite
Minerals reported to co-occur with ferrosilite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Fe₂Si₂O₆
- Mohs hardness
- 5-6
- Density
- 3.96 g/cm³
- Colors
- Streak
- Pale brownish gray
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Anhedral grains
- Cleavage
- Good on {210}, (210) ^ (210) ~88°; partings on {100}, {010}
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- In medium- to high-grade metamorphosed iron formations
Where rockhounds find ferrosilite
Classic worldwide localities
- Copper Mountain and Carmichael Creek, in the Tobacco Root Mountains, Montana, USA
- Ruby Creek area, in the Gravelly Range, Madison Co., Montana, USA
- Carter Creek area, in the Ruby Mountains, Beaverhead and Madison Cos., Montana, USA
- Hanksville, Wayne Co., Utah, USA
- Bauchi, Nigeria
- Tiris, Mauritania
Field-hunting tip
Where it forms: In medium- to high-grade metamorphosed iron formations. If you start seeing magnetite, hematite, ferroan diopside in float, you are in the right ground. Typical habit: anhedral grains. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ferrosilite?+
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What rocks look like ferrosilite?+
What minerals are found with ferrosilite?+
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Find ferrosilite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
