Identification guide · Silicate Minerals
How to Identify Phlogopite
Phlogopite is usually brown, yellow, and tan with a pearly luster and is translucent. It has a Mohs hardness of 2.75. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Its streak is white. It is most often confused with Pyrophyllite, Biotite, and Muscovite; the tests below separate them.
Phlogopite identification checklist
| Color | Brown, Yellow, Tan, Reddish brown, Green | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Pearly | Compare a fresh surface with glass and metal; this luster is softer than either. |
| Transparency | Translucent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 2.75 | A copper coin (Mohs 3.5) scratches it, but a fingernail does not. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect basal | |
| Fracture | Uneven | |
| Specific gravity | 2.8 | Ordinary rock-like heft (specific gravity about 2.8), similar to quartz. |
| Crystal system | Monoclinic |
Phlogopite look-alikes and how to tell them apart
Phlogopite is harder (Mohs 2.75) than Pyrophyllite (Mohs 1.5), so Phlogopite will scratch Pyrophyllite and not the other way round. A fingernail (Mohs 2.5) scratches it easily. Phlogopite vs Pyrophyllite →
Both sit near Mohs 2.75, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Phlogopite vs Biotite →
Both sit near Mohs 2.75, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Phlogopite vs Muscovite →
Mimetite is harder (Mohs 3.75) than Phlogopite (Mohs 2.75), so Mimetite will scratch Phlogopite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Phlogopite vs Mimetite →
Field tips
- 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.
Phlogopite identification FAQ
How can you tell if a rock is Phlogopite?
Check the color (brown, yellow, tan, and reddish brown), the pearly luster, a Mohs hardness of 2.75 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Phlogopite?
Phlogopite has a Mohs hardness of 2.75. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
What is the difference between Phlogopite and Pyrophyllite?
Phlogopite (Mohs 2.75, pearly luster) and Pyrophyllite (Mohs 1.5, pearly luster) look alike, but hardness, streak and heft usually separate them. See the full Phlogopite vs Pyrophyllite comparison for a side-by-side table.
Is Phlogopite worth anything?
Entry-level Phlogopite specimens sell for around $5, typical collector pieces about $40, and premium examples $250 or more depending on size, quality and locality.
What color is Phlogopite?
Phlogopite is most often brown, yellow, tan, and reddish brown. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
