- Stage
- 030.0°
- Field
- —
- Light
- crossed polars
Slide 04 / 09 · Calcite marble
Carrara marble
Michelangelo’s stone. A Jurassic limestone that was buried, heated and squeezed until every grain of calcite recrystallised into an interlocking mosaic.
Calcite has the strongest birefringence in the tray: at 30 µm its retardation passes 5,000 nm, far beyond the coloured orders, so most grains show a soft “high-order white”. Colour survives only at the edges, where grains thin towards their boundaries, and as rainbow fringes along twin lamellae. Watch the outlines in plane light as the stage turns: they strengthen and fade, because calcite’s two refractive indices sit either side of the resin’s.
Under the microscope
What to look for
- 01 · crossed polars · ×4High-order whiteRetardation so high that every colour of the spectrum is present at once, leaving a pale cream.
- 02 · plane light · ×4TwinklingPlane light: turn the stage and grain outlines pulse. It is the classic test for carbonate.
- 03 · crossed polars · ×10Deformation twinsThin lamellae across a grain, left when the marble was strained.
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 |
|---|---|---|---|---|
| calcite | 99% | 0.172 | 5,160 nm | high-order white |
| dolomite, mica, pyrite | 1% | — | 0 nm | dark at every angle |
Γ (retardation) = thickness × birefringence δ. Swatches are computed by the same optics that draw the microscope.