Phyllite
Phyllite is a fine-grained metamorphic rock that represents an intermediate grade between low-grade slate and higher-grade schist, formed by the continued metamorphism of shale or slate under increasing heat and pressure. Its surfaces show a distinctive silky, satiny sheen produced by microscopic mica and chlorite crystals just large enough to reflect light but too small to see individually. Phyllite typically splits along wavy, crenulated foliation surfaces rather than the perfectly flat planes of slate.
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Phyllite properties at a glance
| Type | Metamorphic Rocks |
|---|---|
| Mohs hardness | 2.5–4 |
| Luster | Silky to satiny (sericite/chlorite sheen) |
| Fracture | Splintery along foliation, otherwise uneven |
| Specific gravity | 2.7–2.9 |
| Transparency | Opaque |
| Common colors | Silvery gray, Greenish gray, Black |
| Rarity | common |
Geology & background
Phyllite forms through regional metamorphism when slate is subjected to further increases in temperature and pressure, causing microscopic mica flakes to grow slightly larger and align more strongly than in slate, though not yet large enough to be individually visible as they are in schist. This progressive sequence, from shale to slate to phyllite to schist, reflects a continuous increase in metamorphic grade and is exposed in mountain belts such as the Appalachians, where all four rock types can sometimes be traced across a single traverse.
Rockhounds recognize phyllite by tilting a sample under light to catch its characteristic silky sheen, a quick field test that separates it from the duller slate below it in grade and the coarser, visibly micaceous schist above it. Its foliation surfaces are often gently wavy or crenulated rather than perfectly planar, another useful distinguishing feature. Because phyllite splits fairly easily along foliation, it has occasionally served as a low-grade roofing or flagstone material where slate is unavailable.
How to identify Phyllite
Phyllite displays a fine-grained, silky to satiny sheen on its foliation surfaces caused by aligned microscopic mica, with colors typically gray, greenish, or silvery, and a hardness around 3 to 4 along the foliation, though harder across it. It splits into sheets along gently wavy or crenulated surfaces rather than the flat planes of slate or the coarse layers of schist. Streak is generally pale and unremarkable. Phyllite sits between slate and schist in grain size and metamorphic grade, and is distinguished from slate primarily by its lustrous sheen, which slate lacks.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Phyllite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Phyllite worth?
| Entry-level specimen | $1 |
|---|---|
| Typical collector specimen | $10 |
| Premium specimen | $75+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Field identification
- Metamorphic history study
- Decorative or building stone when suitable
- Education
Interesting facts
- Phyllite occupies the middle position in the classic shale-to-slate-to-phyllite-to-schist metamorphic grade sequence taught in introductory geology.
- Its diagnostic silky sheen comes from countless microscopic mica flakes too small to see individually but large enough to reflect light.
- Phyllite's foliation surfaces are often subtly wavy or crenulated, unlike the perfectly flat cleavage planes typical of slate.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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