- Stage
- 034.0°
- Field
- —
- Light
- crossed polars
Slide 09 / 09 · Carbonaceous chondrite (CV3)
Allende meteorite
More than two tonnes of it fell over northern Mexico one February night in 1969. Its white inclusions are the oldest dated solids in the solar system.
The round objects are chondrules: droplets of molten rock that cooled in space before the planets formed. Some are porphyritic, with olivine crystals set in glass; some are barred, a single olivine crystal grown as parallel plates, so every bar goes dark at the same angle. Radiating pyroxene fans, dark fine-grained rims and the irregular white calcium–aluminium-rich inclusions (whose melilite shows an anomalous blue) sit in a black, sooty matrix.
Under the microscope
What to look for
- 01 · crossed polars · ×10Barred olivine chondruleOne crystal in parallel bars inside an olivine shell: find the angle where every bar is dark.
- 02 · crossed polars · ×10Calcium–aluminium inclusionWhite in plane light; melilite inside it shows an anomalous inky blue between crossed polars.
- 03 · crossed polars · ×4Porphyritic chondrulesOlivine crystals in glass: vivid colours in a dark rim.
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 |
|---|---|---|---|---|
| matrix (fine olivine, sulphides) | 40% | 0.012 | 360 nm | first-order yellow–red |
| chondrules | 45% | 0.036 | 1,080 nm | second order |
| Ca–Al-rich inclusions | 8% | 0.006 | 180 nm | first-order grey–white |
| metal, sulphide | 7% | — | 0 nm | dark at every angle |
Γ (retardation) = thickness × birefringence δ. Swatches are computed by the same optics that draw the microscope.