To name a metamorphic rock, ask two questions. Is it foliated, with minerals lined up in layers or bands? And what was its parent rock? Metamorphism rebuilds the minerals but mostly keeps the chemistry, so the parent shows through. Metamorphic grade names the foliated rocks; the nonfoliated ones are named by their main mineral.
| Parent rock | Metamorphic rock | Foliated? |
|---|---|---|
| Shale | slate → phyllite → schist → gneiss | yes |
| Granite | gneiss | yes (banded) |
| Basalt | greenschist, then amphibolite | weakly |
| Limestone | marble | no |
| Quartz sandstone | quartzite | no |
| Bituminous coal | anthracite | no |
| Any rock baked beside magma | hornfels | no |
Why it works
Foliation needs two things: platy or long minerals (micas, amphiboles) and directed stress to line them up. Shale is full of clay that turns into mica, so it foliates readily. Limestone is almost pure calcite and quartz sandstone almost pure quartz; neither makes platy minerals, so their crystals simply grow into an interlocking mosaic: marble and quartzite. In quartzite the grains are welded so tightly that a break runs straight through them; in sandstone, a break runs around the grains through the weaker cement.
Key idea
Name a metamorphic rock by foliation and parent: foliated slate, phyllite, schist and gneiss; nonfoliated marble (from limestone), quartzite (from quartz sandstone) and hornfels (baked rock).
Worked examples
Worked example: Marble or quartzite?
A pale rock of interlocking crystals fizzes when a drop of dilute acid touches it. Name it and its parent.
- Fizzing in acid means calcite.
- Interlocking calcite crystals: marble, from limestone.
Answer: Marble, from limestone.
Worked example: From the parent
What does quartz sandstone become when metamorphosed?
- It is nearly all quartz, which forms no platy minerals.
- The grains recrystallize and fuse: quartzite, a nonfoliated rock that scratches glass.
Answer: Quartzite.
Common mistake
Quartzite and sandstone look alike: both are pale and made of quartz. The test is how they break. Sandstone breaks around its grains, and loose grains rub off on your fingers. Quartzite breaks right through its grains with a glassy surface, because metamorphism welded them together. Check with a hammer, or look closely at a broken edge.