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 6-7, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: greenish, grayish.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to nearly adamantine on cleavages.
- 4Match the color rangeCompare against the expected color range: pistachio green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: granular, scaly.
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 6-7 vs. 2-2.5); streak differs, so Ottrélite leaves greenish, grayish, Chlorite leaves white; luster reads vitreous to nearly adamantine on cleavages on Ottrélite and pearly on Chlorite.
How to tell apart: Ottrélite is noticeably harder (Mohs 6-7 vs. 2-3); streak differs, so Ottrélite leaves greenish, grayish, Mica leaves white; luster reads vitreous to nearly adamantine on cleavages on Ottrélite and pearly on Mica.
How to tell apart: Streak differs, so Ottrélite leaves greenish, grayish, Staurolite leaves white to grayish; luster reads vitreous to nearly adamantine on cleavages on Ottrélite and subvitreous 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
- MnAl₂O(SiO₄)(OH)₂
- Mohs hardness
- 6-7
- Density
- 3.52 g/cm³
- Colors
- Pistachio green
- Streak
- Greenish, grayish
- Luster
- Vitreous to nearly adamantine on cleavages
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Granular, scaly
- Cleavage
- {001}, perfect; {110}, likely
- Rarity
- Uncommon
- Uses
- Collector, Scientific research
- Host rock
- Formed under nonshearing stress conditions in veins and cavities within low- to medium-grade metamorphic rocks; also a medium-temperature hydrothermal alteration mineral
Where rockhounds find ottrélite
1 mapped spotsClassic worldwide localities
- Ardennes Mountains, from Ottré, Salmchâteau, and Vielsalm, Belgium
Field-hunting tip
Where it forms: Formed under nonshearing stress conditions in veins and cavities within low- to medium-grade metamorphic rocks; also a medium-temperature hydrothermal alteration mineral. If you start seeing dickite, kaolinite, rutile in float, you are in the right ground. Typical habit: granular, scaly. 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.
More ottrélite photos
Common questions
How do you identify ottrélite?+
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What rocks look like ottrélite?+
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Find ottrélite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
