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 1-2, 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: waxy, resinous, dull.
- 4Match the color rangeCompare against the expected color range: greenish yellow, brownish green, olive green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: earthy.
Often confused with
Nontronite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads waxy, resinous, dull on Nontronite and pearly on Chlorite.
How to tell apart: Streak differs, so Nontronite leaves white, Glauconite leaves light green; luster reads waxy, resinous, dull on Nontronite and dull, glistening on Glauconite.
How to tell apart: Luster reads waxy, resinous, dull on Nontronite and dull, earthy on Montmorillonite.
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
- 1-2
- Density
- 2.2-2.3 g/cm³
- Streak
- White
- Luster
- Waxy, resinous, dull
- Transparency
- Translucent to nearly opaque
- Crystal system
- Monoclinic
- Crystal habit
- Earthy
- Cleavage
- {001}, perfect
- Rarity
- Common
- Uses
- Collector, Scientific research
- Host rock
- A weathering product of basalts, kimberlites, and other ultramafic igneous rocks; in poorly-drained volcanic ash soils
Where rockhounds find nontronite
Classic worldwide localities
- Spruce Pine district, Mitchell Co., North Carolina, USA
- Colfax and Garfield, Whitman Co., Washington, USA
- Petaluma, Sonoma Co., Crestmore, Riverside Co., and near Woody, Kern Co., California, USA
- Santa Rita, Grant Co., New Mexico, USA
- Saint-Pardoux, Dordogne, France
- Froland, Norway
Field-hunting tip
Where it forms: A weathering product of basalts, kimberlites, and other ultramafic igneous rocks; in poorly-drained volcanic ash soils. If you start seeing quartz, opal, hornblende in float, you are in the right ground. Typical habit: earthy. 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.
