Cookeite is a lithium-rich member of the chlorite group that typically forms as a coating or alteration product on other minerals like tourmaline. It is most often found as thin, pearly, micaceous aggregates or soft, pseudo-hexagonal plates in pockets of complex pegmatites.
Is this cookeite?
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 cookeite with a known reference. Cookeite sits at Mohs 2.5-3.5, 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 or silky.
- 4Match the color rangeCompare against the expected color range: white, green, brown, yellowish white, pinkish white.
- 5Look at form & habitCrystal system: triclinic. Typical habit: micaceous, pseudo hexagonal.
Often confused with
Cookeite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside cookeite
Minerals reported to co-occur with cookeite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- LiAl₄(Si₃Al)O₁₀(OH)₈
- Mohs hardness
- 2.5-3.5
- Density
- 2.58-2.69 g/cm³
- Streak
- White
- Luster
- Pearly or silky
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
- Crystal habit
- Micaceous, pseudo hexagonal
- Cleavage
- {001}, perfect
- Rarity
- Uncommon
- Uses
- Collector, Mineralogical study
- Host rock
- A late-stage hydrothermal alteration product of lithium-bearing minerals in pegmatites; a primary hydrothermal vein mineral
Where rockhounds find cookeite
1 mapped spotsClassic worldwide localities
- Hebron, Mt. Mica, near Paris, and Buckfield, Oxford Co., Maine, USA
- Pala and Rincon districts, San Diego Co., California, USA
- Jeffrey quarry, North Little Rock, Pulaski Co., Arkansas, USA
- Waitabit Creek, northwest of Donald, British Columbia, Canada
- Ogofau, Carmarthenshire, England
- Radkovice and Dobrá Voda, Czech Republic
Field-hunting tip
Where it forms: A late-stage hydrothermal alteration product of lithium-bearing minerals in pegmatites; a primary hydrothermal vein mineral. If you start seeing lepidolite, spodumene, tourmaline in float, you are in the right ground. Typical habit: micaceous, pseudo hexagonal. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in California. Start trip planning there.
Common questions
How do you identify cookeite?+
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Find cookeite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
