Montdorite is a rare phyllosilicate mineral belonging to the mica group, primarily found in the volcanic rocks of the Mont-Dore massif in France. It typically occurs as small, delicate, yellow to brown platy crystals that form within vugs or cavities of volcanic host rock.
Is this montdorite?
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 montdorite with a known reference. Montdorite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: pearly.
- 4Match the color rangeCompare against the expected color range: green, brownish green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: microscopic crystals.
Often confused with
Montdorite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside montdorite
Minerals reported to co-occur with montdorite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KFe₁.₅Mn₀.₅Mg₀.₅Si₄O₁₀(F,OH)₂
- Mohs hardness
- 2.5
- Density
- 3.02 g/cm³
- Colors
- Streak
- White
- Luster
- Pearly
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Microscopic crystals
- Cleavage
- Perfect basal
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Small grains in perialkaline rhyolite
Where rockhounds find montdorite
Classic worldwide localities
- Mt. Dore stratovolcano, near La Bourboule, France (type locality)
Field-hunting tip
Where it forms: Small grains in perialkaline rhyolite. If you start seeing sanidine, tridymite, magnetite in float, you are in the right ground. Typical habit: microscopic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify montdorite?+
Where is montdorite found?+
What rocks look like montdorite?+
What minerals are found with montdorite?+
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Find montdorite on the map
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