Phlogopite
Phlogopite is a magnesium-rich member of the mica group, recognized by its brown to tan color and perfect basal cleavage that splits into thin, flexible, elastic sheets. It typically forms tabular or pseudohexagonal crystals in marbles and magnesium-rich rocks.
Phlogopite properties at a glance
| Type | Silicate Minerals |
|---|---|
| Chemical formula | KMg3(AlSi3O10)(OH)2 |
| Mohs hardness | 2.75 |
| Crystal system | monoclinic |
| Luster | pearly |
| Streak | White |
| Cleavage | Perfect basal |
| Fracture | Uneven |
| Specific gravity | 2.8 |
| Transparency | translucent |
| Common colors | Brown, Yellow, Tan, Reddish brown, Green |
| Rarity | common |
Geology & background
Phlogopite forms in magnesium-rich metamorphic rocks such as marble and skarn, and in ultramafic and alkaline igneous rocks, where potassium and magnesium combine in mica-forming conditions.
Phlogopite and related sheet micas were historically mined for electrical and thermal insulation, lamp chimneys, and stove windows, valued because thin sheets are transparent, flexible, and resistant to heat.
How to identify Phlogopite
- Look for brown to tan color and a perfect basal cleavage that peels into thin, flexible, elastic sheets.
- Single cleavage flakes are nearly transparent and bounce back after gentle bending.
- Phlogopite is soft and commonly occurs in marble or magnesium-rich rock, often with calcite and diopside.
Where Phlogopite is found
Phlogopite occurs in marbles, skarns, and magnesium-rich igneous rocks worldwide, with notable specimen localities in Canada, Russia, Madagascar, Sri Lanka, the United States, and Italy.
How much is Phlogopite worth?
| Entry-level specimen | $5 |
|---|---|
| Typical collector specimen | $40 |
| Premium specimen | $250+ |
Value depends on crystal size, color, clarity of cleavage sheets, locality, matrix, and overall condition.
Common uses
- Electrical insulation
- Thermal insulation
- Filler in paints and plastics
- Industrial mineral applications
- Collector specimens
Interesting facts
- Phlogopite cleaves into thin sheets so flexible and elastic that they spring back after bending.
- Its name comes from a Greek word for fire-like, a reference to the reddish tints seen in some crystals.
- Like other micas, it can be heat-resistant and electrically insulating, which gave sheet mica industrial value.
- Phlogopite grades chemically into biotite as iron replaces magnesium in its structure.
Care & safety
Phlogopite is soft and its thin cleavage sheets bend and split easily, so crystals are fragile.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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