Shap granite

Shap, Cumbria
Early Devonian, c. 400 million years

A pink granite quarried above Shap since the 1860s and polished for columns and memorials. Its big feldspar crystals are the first thing you see in the quarry face; under the microscope they turn out to be full of other minerals.

Between crossed polars the quartz is grey to white and never coloured: at 30 µm it cannot rise above first-order white. The pink megacrysts are orthoclase, split in two halves by a Carlsbad twin and laced with thin films of albite (perthite). Plagioclase shows its stripes, and the dark biotite flakes change from straw to deep brown as the stage turns.

Under the microscope

What to look for

01 · crossed polars · ×10Undulose extinctionA quartz grain that goes dark in a sweeping shadow instead of all at once: its lattice was bent after it grew.
02 · crossed polars · ×4Carlsbad twinThe megacryst is two crystals sharing a plane: turn the stage and one half darkens while the other stays grey.
03 · plane light · ×10Pleochroic haloesDark rings in biotite around tiny zircons. Radiation from uranium in the zircon damaged the mica around it.

The whole section

Plane light | crossed polars

The same slice of rock with the analyser out and in. Drag the line.

PPLXPL 01 · SHAP · 30 µm

Mode, by eye

What it is made of

Mineral	Share	δ	Γ max at 30 µm	Strongest colour
K-feldspar	36%	0.007	210 nm	first-order grey–white
quartz	31%	0.009	270 nm	first-order grey–white
plagioclase	25%	0.009	270 nm	first-order grey–white
biotite	6%	0.055	1,650 nm	third order
muscovite, zircon, oxides	2%	0.042	1,260 nm	third order

Γ (retardation) = thickness × birefringence δ. Swatches are computed by the same optics that draw the microscope.