Nontronite is an iron-rich member of the smectite clay group, typically appearing as soft, yellowish to green, earthy masses. It is commonly found as a secondary mineral resulting from the alteration of basaltic rocks or volcanic glass in hydrothermal environments.
Is this nontronite?
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 nontronite with a known reference. Nontronite sits at Mohs 1.5-2, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Nontronite leaves a yellow streak.
- 3Read the lusterHold the specimen under a strong light. Nontronite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: yellow, green, yellowish-green, brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: massive, earthy, clay-like.
Often confused with
Nontronite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Nontronite leaves yellow, Chlorite leaves white; luster reads dull on Nontronite and pearly on Chlorite.
How to tell apart: Streak differs, so Nontronite leaves yellow, Glauconite leaves pale green.
How to tell apart: Streak differs, so Nontronite leaves yellow, Montmorillonite leaves white.
Often found alongside nontronite
Minerals reported to co-occur with nontronite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Na₀.₃Fe³⁺₂Si₃.₈Al₀.₂O₁₀(OH)₂·nH₂O
- Mohs hardness
- 1.5-2
- Density
- 2.2-2.5 g/cm³
- Streak
- Yellow
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Massive, Earthy, Clay-like
- Cleavage
- Perfect Basal
- Rarity
- Common
- Uses
- Collector, Scientific Research
- Host rock
- Weathered Basalt, Hydrothermal Veins
- Typical price
- $5-30 for small samples
Where rockhounds find nontronite
Classic worldwide localities
- Nontron, France
- Hohenstein, Germany
- Columbia River Basalt Group, USA
- Cripple Creek, USA
Field-hunting tip
Look in weathered basalt, hydrothermal veins country, the host setting where nontronite typically forms. If you start seeing quartz, opal, calcite in float, you are in the right ground. Field specimens usually show a massive, earthy, clay-like habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find nontronite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
