Ottrelite is a member of the chloritoid group frequently found in low-to-medium grade metamorphic rocks such as phyllites and schists. It is characterized by its dark, tabular, micaceous crystals that often appear as small black spots or 'porphyroblasts' within the rock matrix.
Is this ottrélite?
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 ottrélite with a known reference. Ottrélite sits at Mohs 6.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Ottrélite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Ottrélite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: dark gray, dark green, black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: tabular crystals, platy, foliated, massive.
Often confused with
Ottrélite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Ottrélite is noticeably harder (Mohs 6.5 vs. 2-2.5); luster reads vitreous on Ottrélite and pearly on Chlorite.
How to tell apart: Ottrélite is noticeably harder (Mohs 6.5 vs. 2-3); luster reads vitreous on Ottrélite and pearly on Mica.
How to tell apart: Luster reads vitreous on Ottrélite and vitreous to resinous on Staurolite.
Often found alongside ottrélite
Minerals reported to co-occur with ottrélite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Fe,Mg,Mn)₂Al₄Si₂O₁₀(OH)₄
- Mohs hardness
- 6.5
- Density
- 3.5-3.6 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Tabular Crystals, Platy, Foliated, Massive
- Cleavage
- Perfect On {001}
- Rarity
- Uncommon
- Uses
- Collector, Scientific Research
- Host rock
- Metamorphic Rocks
- Typical price
- $10-100 per specimen
Where rockhounds find ottrélite
1 mapped spotsClassic worldwide localities
- Belgium
- France
- Austria
- United States
Field-hunting tip
Look in metamorphic rocks country — that is the host setting where ottrélite typically forms. If you start seeing quartz, muscovite, chlorite in float, you are in the right ground. Field specimens usually show a tabular crystals, platy, foliated, massive habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in North Carolina — start trip planning there.
Common questions
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Find ottrélite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
