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 — Nontronite leaves yellow, Chlorite leaves white; luster reads dull on Nontronite and pearly on Chlorite.
How to tell apart: Streak differs — Nontronite leaves yellow, Glauconite leaves pale green.
How to tell apart: Streak differs — 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 — that is 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.
