Brownmillerite is a rare calcium iron aluminate mineral typically found in high-temperature metamorphic environments like coal-seam burn sites. It is notable for its role in the chemistry of Portland cement clinker and usually appears as dark, opaque grains in massive aggregates.
Is this brownmillerite?
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 brownmillerite with a known reference. Brownmillerite 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. Brownmillerite leaves a brown streak.
- 3Read the lusterHold the specimen under a strong light. Brownmillerite typically shows a submetallic luster.
- 4Match the color rangeCompare against the expected color range: dark brown, brownish black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: massive, granular, or as interstitial grains.
Often confused with
Brownmillerite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Brownmillerite leaves brown, Magnetite leaves black; luster reads submetallic on Brownmillerite and metallic on Magnetite.
How to tell apart: Streak differs — Brownmillerite leaves brown, Iron Ore leaves reddish-brown to black; luster reads submetallic on Brownmillerite and metallic to submetallic on Iron Ore.
Often found alongside brownmillerite
Minerals reported to co-occur with brownmillerite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₂FeAlO₅
- Mohs hardness
- 5-6
- Density
- 4.26 g/cm³
- Colors
- Streak
- Brown
- Luster
- Submetallic
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Massive, Granular, Or as Interstitial Grains
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Scientific Research
- Host rock
- Metamorphic Rocks Formed By Combustion or High-temperature Contact Metamorphism
- Typical price
- $50-300 per specimen
Where rockhounds find brownmillerite
Classic worldwide localities
- Hatrurim Formation, Israel
- Bellerberg Volcano, Germany
- Kopeysk, Russia
- Iron Mountains, USA
Field-hunting tip
Look in metamorphic rocks formed by combustion or high-temperature contact metamorphism country — that is the host setting where brownmillerite typically forms. If you start seeing gehlenite, larnite, periclase in float, you are in the right ground. Field specimens usually show a massive, granular, or as interstitial grains habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find brownmillerite on the map
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