- Stage
- 000.0°
- Field
- —
- Light
- crossed polars
Slide 05 / 09 · Oolitic limestone
Portland oolite
The stone of St Paul’s Cathedral. It is made of ooids: tiny spheres of calcite that grew layer by layer as they rolled in warm, shallow sea water.
Each ooid has a nucleus (a sand grain or a scrap of shell) wrapped in concentric laminae of calcite crystals radiating outwards. Because every radius points a different way, each ooid shows a dark cross that stays aligned with the polarisers however far you turn the stage. We impregnate porous rocks with blue resin before cutting, so the open pore space shows as bright blue in plane light.
Under the microscope
What to look for
- 01 · crossed polars · ×4Extinction crossesTurn the stage: the ooids turn with it, but their dark crosses stay put.
- 02 · plane light · ×4Blue resin poresPlane light shows the pore space, dyed blue when the stone was impregnated.
- 03 · crossed polars · ×10Fringe cementA rim of bladed calcite crystals that grew on each ooid after it was buried.
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 |
|---|---|---|---|---|
| ooids (calcite) | 72% | 0.172 | 5,160 nm | high-order white |
| sparry calcite cement | 16% | 0.172 | 5,160 nm | high-order white |
| pore space (blue resin) | 10% | — | 0 nm | dark at every angle |
| shell fragments, quartz | 2% | 0.009 | 270 nm | first-order grey–white |
Γ (retardation) = thickness × birefringence δ. Swatches are computed by the same optics that draw the microscope.