- Stage
- 040.0°
- Field
- —
- Light
- crossed polars
Slide 02 / 09 · Quartz dolerite
Whin Sill dolerite
The sheet of dark rock that Hadrian’s Wall stands on. It was injected as magma between layers of sediment and cooled slowly enough to grow crystals you can see with a hand lens.
Laths of plagioclase, each striped with albite twins, lie at every angle. Augite grew later and wrapped around them in large single crystals: turn the stage and one whole plate of augite goes dark at once, with the laths still bright inside it. That texture is called ophitic. The black shapes are iron–titanium oxides; the green pockets are chlorite, which turns an odd inky blue between crossed polars.
Under the microscope
What to look for
- 01 · crossed polars · ×4Ophitic textureOne augite crystal enclosing many plagioclase laths: find the rotation where the whole plate is dark.
- 02 · crossed polars · ×10Albite twinsStripes along each lath. Neighbouring stripes go dark at equal and opposite angles either side of the lath.
- 03 · plane light · ×10ChloritePale green in plane light, an anomalous blue-grey between crossed polars.
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 |
|---|---|---|---|---|
| plagioclase | 48% | 0.009 | 270 nm | first-order grey–white |
| augite | 38% | 0.024 | 720 nm | second order |
| Fe–Ti oxides | 6% | — | 0 nm | dark at every angle |
| chlorite, granophyre | 8% | 0.003 | 90 nm | first-order grey–white |
Γ (retardation) = thickness × birefringence δ. Swatches are computed by the same optics that draw the microscope.