Spotted Slate
Spotted slate, or spotted phyllite, is a fine-grained, cleaved rock speckled with small dark knots. It forms in the outer zone of the baked halo, called an aureole, around an igneous intrusion, where heat has begun to grow new minerals in an existing slate.
Spotted slate, or spotted phyllite, is a fine-grained, cleaved rock speckled with small dark knots. It forms in the outer zone of the baked halo, called an aureole, around an igneous intrusion, where heat has begun to grow new minerals in an existing slate.
🪨 Geology Guide Specimen: Open Spotted Slate / Phyllite 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.
Spots: What Are the Dark Knots?
The spots, a millimeter or two across, are the first stage of new crystals of cordierite or andalusite, or sometimes clusters of carbon and iron minerals. They form from the alumina and iron in the slate and grow without fully organizing, so they appear as oval blotches called maculae, on a cleaved matrix that is otherwise unchanged.
Contact Aureoles: How Does Heat Zone the Rock?
Heat from a pluton fades with distance. Close to a granite the host rock is baked into hornfels, farther out into rock with andalusite or cordierite crystals, and at the margin it is only spotted. Mapping these zones shows how far the heat reached and the size of the intrusion.
Contact versus Regional: Why Does It Differ from Slate?
Slate gets its cleavage from directed pressure in mountain belts. Spotting is thermal, from heat alone, and it overprints an existing cleavage, so the same rock can record both mountain building and a later intrusion of granite.
Spotted Slate at a Glance
| Property | Value |
|---|---|
| Type / variety | Cordierite-Porphyroblast Outer-Aureole Slate |
| Composition | Fine Micaceous Cleaved Matrix + Millimeter-Sized Dark Cordierite/Andalusite Spots |
| Geology Guide group | Metamorphic Rocks (Foliated, Granoblastic & High-P) |
| Rock Classification | Low-Grade Foliated (Slate & Phyllite) |
| Mohs hardness | Mohs 3.5–4.5 |
| Texture / habit | Planar or wavy cleavage speckled with dark ovoid incipient porphyroblasts |
| Density | 2.74–2.82 g/cm³ |
| Lustre and cleavage | Silky grey cleavage plane dotted with dark 'grain-like' knots |
| Geological setting | Outer thermal aureoles where granite plutons intrude cleaved regional pelites |
| Diagnostic feature (Geology Guide) | Finely cleaved grey-to-silky rock peppered with dark, millimeter-sized oval knots ('maculae') of embryonic cordierite and andalusite that began nucleating at the outer edge of a magma body's heat zone. |
| Typical spots | Cordierite or andalusite, 1-2 mm |
| Setting | Outer contact aureole |