- Stage
- 020.0°
- Field
- —
- Light
- crossed polars
Slide 08 / 09 · Coarse feldspathic sandstone
Millstone Grit
The coarse sandstone of the edges west of Sheffield, once cut into millstones and grindstones for the city’s blade trade. Our local rock.
Sand carried south by rivers from mountains in the north: mostly quartz, with microcline in tartan twinning, cloudy orthoclase and scraps of older rocks. Many quartz grains have grown overgrowths in the same crystal orientation, so they go dark with their grain; a line of dust marks the old surface. The blue is pore space filled with dyed resin, which is how much water, or gas, the stone could hold.
Under the microscope
What to look for
- 01 · crossed polars · ×10Tartan twinningMicrocline: two sets of twins at right angles, blurred like a woven check.
- 02 · plane light · ×10Dust rimsA line of specks inside the grain: the surface before the overgrowth. In plane light.
- 03 · plane light · ×4PorosityThe blue resin shows the connected pore space between the grains.
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 | 66% | 0.009 | 270 nm | first-order grey–white |
| K-feldspar | 17% | 0.007 | 210 nm | first-order grey–white |
| pore space (blue resin) | 9% | — | 0 nm | dark at every angle |
| rock fragments, kaolinite, iron oxide | 8% | 0.020 | 600 nm | second order |
Γ (retardation) = thickness × birefringence δ. Swatches are computed by the same optics that draw the microscope.