- Stage
- 008.0°
- Field
- —
- Light
- crossed polars
Slide 06 / 09 · Garnet–mica schist
Glen Clova garnet schist
From the glens where George Barrow mapped, in 1893, how mud turns into schist as it is buried deeper and heated: the first metamorphic zones ever drawn.
Garnet is cubic, so it is dark between crossed polars at every angle; only its quartz inclusions show, in curved trails that record the crystal rotating as it grew. Around it the micas are aligned in a foliation that flows round the garnet like water round a stone. Turn the stage until that foliation lies along a polariser and every mica in the field goes dark together.
Under the microscope
What to look for
- 01 · crossed polars · ×4Isotropic garnetBlack at every rotation between crossed polars. Pale pink in plane light.
- 02 · crossed polars · ×10Snowball trailsQuartz inclusions in curved trails: the garnet rotated as it grew.
- 03 · crossed polars · ×4Micas go dark togetherAt 0° the foliation lies along the polariser and the whole mica matrix extinguishes.
The whole section
Plane light | crossed polars
The same slice of rock with the analyser out and in. Drag the line.
Mode, by eye
What it is made of
| Mineral | Share | δ | Γ max at 30 µm | Strongest colour |
|---|---|---|---|---|
| quartz | 34% | 0.009 | 270 nm | first-order grey–white |
| muscovite | 26% | 0.042 | 1,260 nm | third order |
| biotite | 18% | 0.055 | 1,650 nm | third order |
| garnet | 14% | — | 0 nm | dark at every angle |
| plagioclase, oxides | 8% | 0.009 | 270 nm | first-order grey–white |
Γ (retardation) = thickness × birefringence δ. Swatches are computed by the same optics that draw the microscope.