Phlogopite is a magnesium-rich mica commonly found in contact-metamorphosed limestones and ultramafic igneous rocks. It is easily identified by its distinctive brownish color and perfect, flexible micaceous cleavage that allows it to peel into thin, elastic sheets.
Is this phlogopite?
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 phlogopite with a known reference. Phlogopite sits at Mohs 2-3, 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 to submetallic on cleavage.
- 4Match the color rangeCompare against the expected color range: brown, yellowish-brown, reddish-brown, greenish-brown, colorless, gray, green, yellow.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: lamellar, micaceous, scaly.
Often confused with
Phlogopite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads pearly to submetallic on cleavage on Phlogopite and splendent to submetallic, vitreous, pearly on cleavage on Biotite.
How to tell apart: Luster reads pearly to submetallic on cleavage on Phlogopite and vitreous to pearly or silky on Muscovite.
Often found alongside phlogopite
Minerals reported to co-occur with phlogopite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KMg₃(AlSi₃O₁₀)(OH)₂
- Mohs hardness
- 2-3
- Density
- 2.78-2.85 g/cm³
- Streak
- White
- Luster
- Pearly to submetallic on cleavage
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
- Crystal habit
- Lamellar, micaceous, scaly
- Cleavage
- {001}, perfect
- Fluorescence
- Straw to Lemon Yellow under SW UV; Weak Blue-white to blue-gray under LW UV
- Rarity
- Common
- Uses
- Industrial, Insulation, Collector
- Host rock
- In metamorphosed dolostones and magnesium-rich limestones; in ultramafic rocks as kimberlites, peridotites, lamproites, and serpentinites
Where rockhounds find phlogopite
1 mapped spotsClassic worldwide localities
- Antwerp and Natural Bridge, Jefferson Co., and Edwards and Pierrepont, St. Lawrence Co., New York, USA
- Franklin, Sussex Co., New Jersey, USA
- Lacey mine, Frontenac Co., and in North and South Burgess Townships, Ontario, Canada
- Perkin's Mills, in Gatineau Co., Quebec, Canada
- Slyudyanka region, near Lake Baikal, Siberia, Russia
- Campolungo, near St. Gotthard, Ticino, Switzerland
Field-hunting tip
Where it forms: In metamorphosed dolostones and magnesium-rich limestones; in ultramafic rocks as kimberlites, peridotites, lamproites, and serpentinites. If you start seeing dolomite, calcite, diopside in float, you are in the right ground. Typical habit: lamellar, micaceous, scaly. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in New York. Start trip planning there.
Common questions
How do you identify phlogopite?+
Where is phlogopite found?+
Can I find phlogopite in the United States?+
What rocks look like phlogopite?+
What minerals are found with phlogopite?+
What kind of rock does phlogopite form in?+
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Find phlogopite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
