RocksdayMylonite vs Slate
How to tell them apart
A fine-grained, streaky rock formed deep in fault zones where rocks flow instead of breaking.
Photo: Unknown, CC BY-SA 1.0
Shale transformed by low-grade metamorphism into a hard rock that splits into smooth, flat sheets.
Photo: Dexter Perkins, CC0
Quick tests
- Scratch glass: Mylonite scratches glass (Mohs 6–7); Slate can't (Mohs 3–4).
Key differences
- Slate splits into flat sheets.
Side by side
| Property | Mylonite | Slate |
|---|---|---|
| Type | Metamorphic rock | Metamorphic rock |
| Mohs hardness | 6–7 | 3–4 |
| Luster | Dull to silky | Dull to faint sheen |
| Colors | Gray, Black, Green | Gray, Black, Green, Purple |
| Crystal system / structure | Fine-grained, streaky (sheared) | Very fine, foliated (slaty cleavage) |
| Specific gravity | 2.6–2.9 | 2.7–2.8 |
| Found in | Moine Thrust, Scotland; Alpine Fault, New Zealand | Gwynedd, Wales; Slate Valley, Vermont–New York |
Properties that are the same for both are grayed out.
How to identify mylonite
- Fine streaks and stretched grains.
- Larger crystals smeared into 'eyes'.
- Found along major faults.
Structure: Fine-grained, streaky (sheared)
Full Mylonite guide →How to identify slate
- Splits into thin, flat, smooth sheets.
- Rings when tapped.
- Harder than shale.
- Grains invisible.
Structure: Very fine, foliated (slaty cleavage)
Full Slate guide →More comparisons
Could you tell them apart from a blurry close-up?
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