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. Streak: light brown.
- 3Read the lusterHold the specimen under a strong light. Luster: submetallic.
- 4Match the color rangeCompare against the expected color range: reddish brown.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: platy, square.
Often confused with
Brownmillerite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Brownmillerite leaves light brown, Magnetite leaves black; luster reads submetallic on Brownmillerite and metallic to submetallic, may be dull on Magnetite.
How to tell apart: Streak differs, so Brownmillerite leaves light brown, Hematite leaves cherry-red or reddish brown; luster reads submetallic on Brownmillerite and metallic or submetallic to dull on Hematite.
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
- 3.76 g/cm³
- Colors
- Streak
- Light brown
- Luster
- Submetallic
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy, square
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- In thermally metamorphosed limestone blocks included in volcanic rocks (near Mayen, Germany); in high-temperature, thermally metamorphosed, impure limestones (Hatrurim Formation, Israel)
Where rockhounds find brownmillerite
Classic worldwide localities
- Ettringer-Bellerberg volcano, near Mayen, Eifel district, Germany
- Found at Klöch, Styria, Austria
- Hatrurim Formation, Israel
Field-hunting tip
Where it forms: In thermally metamorphosed limestone blocks included in volcanic rocks (near Mayen, Germany); in high-temperature, thermally metamorphosed, impure limestones (Hatrurim Formation, Israel). If you start seeing calcite, ettringite, wollastonite in float, you are in the right ground. Typical habit: platy, square. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify brownmillerite?+
Where is brownmillerite found?+
What rocks look like brownmillerite?+
What minerals are found with brownmillerite?+
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What is brownmillerite used for?+
Find brownmillerite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
