Illite is a non-expanding, clay-sized mica mineral that is a common component of shales and other fine-grained sedimentary rocks. It typically occurs as earthy, massive aggregates that lack distinct crystal faces visible to the naked eye. Collectors often look for it within sedimentary strata where it serves as a primary constituent of clay minerals.
Is this illite?
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 illite with a known reference. Illite 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: earthy.
- 4Match the color rangeCompare against the expected color range: white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: aggregates.
Often confused with
Illite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads earthy on Illite and vitreous to pearly or silky on Muscovite.
How to tell apart: Luster reads earthy on Illite and dull, earthy on Montmorillonite.
How to tell apart: Luster reads earthy on Illite and pearly to dull earthy on Kaolinite.
Often found alongside illite
Minerals reported to co-occur with illite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂,(H₂O)]
- Mohs hardness
- 1-2
- Density
- 2.6-2.9 g/cm³
- Colors
- Streak
- White
- Luster
- Earthy
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Aggregates
- Cleavage
- Perfect on {001}
- Rarity
- Common
- Uses
- Industrial, Collector
- Host rock
- Weathering or hydrothermal alteration of muscovite-phengite
Where rockhounds find illite
1 mapped spotsClassic worldwide localities
- Maquoketa shale, Gilead, Calhoun County, Illinois, USA (type locality)
Field-hunting tip
Where it forms: Weathering or hydrothermal alteration of muscovite-phengite. If you start seeing quartz, feldspar, calcite in float, you are in the right ground. Typical habit: aggregates. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah. Start trip planning there.
Common questions
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Find illite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
