- Stage
- 012.0°
- Field
- —
- Light
- crossed polars
Slide 03 / 09 · Olivine cumulate (peridotite)
Rùm peridotite
From the layered intrusion that forms the mountains of Rùm: the solidified floor of a magma chamber, where olivine crystals settled and piled up.
Olivine is the showpiece of the tray. Its birefringence is four times that of quartz, so at 30 µm it reaches second- and third-order blues, greens, magentas and yellows. Every grain is cracked, and serpentine has grown into the cracks, a first step towards the rock turning into serpentinite. The small black crystals are chromite, red-brown only where they are thinnest.
Under the microscope
What to look for
- 01 · crossed polars · ×4Third-order coloursA grain cut near the maximum shows colours that repeat: pink and green bands above the second-order blue.
- 02 · crossed polars · ×10Serpentine in cracksPale, fibrous, low-order grey in the fractures. The fibres grow across the crack.
- 03 · plane light · ×10High reliefIn plane light olivine’s outlines and pitted surface stand out: it bends light much more than the resin.
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 |
|---|---|---|---|---|
| olivine | 84% | 0.036 | 1,080 nm | second order |
| augite | 9% | 0.024 | 720 nm | second order |
| plagioclase | 5% | 0.009 | 270 nm | first-order grey–white |
| chromite | 2% | — | 0 nm | dark at every angle |
Γ (retardation) = thickness × birefringence δ. Swatches are computed by the same optics that draw the microscope.