In Metamorphic Rocks you saw shale become slate and granite become gneiss. Now we follow one rock up the ladder of heat and pressure. Metamorphic grade is how intensely a rock has been changed, and as pressure and temperature rise, the same shale passes through a series of new rocks and new minerals.
| Grade | Rock from shale | What you see | Index mineral |
|---|---|---|---|
| Low | slate | dull, fine-grained; splits into flat sheets | chlorite (about 300 °C) |
| Low to medium | phyllite | silky sheen; crinkly sheets | biotite (about 420 °C) |
| Medium | schist | visible, sparkly mica flakes | garnet (about 480 °C), staurolite (about 560 °C) |
| High | gneiss | light and dark bands of coarse crystals | kyanite (about 620 °C), sillimanite (about 680 °C) |
Why it works
Clay minerals in shale are stable only at low temperature. As the rock heats, atoms rearrange into minerals that are stable at the new conditions. First, tiny micas grow, lined up by directed stress (slate). With more heat, they grow larger (phyllite, then schist). At high grade, light minerals (quartz, feldspar) and dark ones (biotite, amphibole) separate into bands (gneiss). Some minerals form only above a certain temperature, so their first appearance marks a grade: these are index minerals, mapped as lines across mountain belts since the 1890s in Scotland.
Key idea
As heat and pressure rise, shale becomes slate, phyllite, schist, then gneiss, and index minerals appear in order: chlorite, biotite, garnet, staurolite, kyanite, sillimanite.
Worked examples
Worked example: Which index mineral?
A shale reached about $520^\circ\text{C}$. Using the table, what is its highest-grade index mineral?
- 520 °C is past garnet (480 °C) but short of staurolite (560 °C).
- So garnet is the highest-grade index mineral: it is a garnet schist.
Answer: Garnet.
Worked example: How deep for garnet?
Garnet appears at about $480^\circ\text{C}$. With a surface of $15^\circ\text{C}$ and a gradient of $30^\circ\text{C/km}$, how deep must shale be buried?
- $480 - 15 = 465^\circ\text{C}$ of warming.
- $465 \div 30 = 15.5\,\text{km}$.
Answer: About $15.5\,\text{km}$.
Common mistake
Slate, phyllite, schist and gneiss are often treated as four unrelated rocks. They can be one parent rock, shale, cooked to higher and higher grade, and in many mountain belts you can walk from one to the next. Check with the micas: their size grows steadily from slate to schist, the clue that it is one sequence.