Slate
Slate is a fine-grained metamorphic rock formed from mudstone or shale at low grade, which splits into smooth, flat plates along a property called slaty cleavage. It is dull, hard and dense, and it rings with a clear 'clink' when tapped.
Slate is a fine-grained metamorphic rock formed from mudstone or shale at low grade, which splits into smooth, flat plates along a property called slaty cleavage. It is dull, hard and dense, and it rings with a clear "clink" when tapped.
🪨 Geology Guide Specimen: Open Roofing Slate in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 6 subgroups of Metamorphic, and test your eye in LithoClade.
Slaty Cleavage: Why Does Slate Split?
Under directed pressure, tiny clay minerals recrystallize into microscopic flakes of mica and chlorite that line up perpendicular to the squeezing. The rock then splits along these planes. The cleavage can cut across the original bedding, which is the way to tell slate from the bedding-parallel splitting of shale.
Low Grade: What Conditions Make It?
Slate forms at the mildest end of regional metamorphism, at a few hundred degrees Celsius, in the folded and thrust-faulted belts of collisional mountain ranges. With more heat and pressure it turns into phyllite, then schist and gneiss, a sequence geologists call increasing metamorphic grade.
Uses: Roofs, Floors and Pool Tables
Slate splits into thin, durable, water-resistant sheets, so it is used for roofing, floors and cladding, and its flat, uniform surfaces made it the material of school blackboards and billiard tables. Welsh, Vermont and Spanish quarries are among the major sources.
Slate at a Glance
| Property | Value |
|---|---|
| Type / variety | Fine-Grained Slaty-Cleavage Metapelite |
| Composition | Microscopic Parallel Illite/Muscovite + Chlorite + Fine Quartz (<0.01 mm) |
| Geology Guide group | Metamorphic Rocks (Foliated, Granoblastic & High-P) |
| Rock Classification | Low-Grade Foliated (Slate & Phyllite) |
| Mohs hardness | Mohs 3–4 |
| Texture / habit | Ultra-fine-grained planar cleavage cutting across original bedding |
| Density | 2.70–2.80 g/cm³ |
| Lustre and cleavage | Dull matte charcoal-grey, purple, or black; rings with a clear 'clink' when tapped |
| Geological setting | Fold-and-thrust mountain belts at 200–320 °C (Shale protolith) |
| Diagnostic feature (Geology Guide) | Compact, matte charcoal-grey rock that splits into billiard-table-flat, rigid plates along parallel tectonic cleavage planes and emits a crisp metallic 'clink' rather than a dull thud when tapped. |
| Protolith | Mudstone or shale |
| Texture | Slaty cleavage |
See Also
- Phyllite — the next step up in grade
- Mudstone — a common parent rock
- Plate Tectonics — the collisions that make slate belts