What you see under the microscope · Petrography

What you see with a polarising microscope

A rock does not let light through unless it is cut very thin. A piece is glued to a glass slide and ground down to 30 µm: at that thickness almost all its minerals are transparent. A polarising microscope has a polariser below the sample, an analyser above it and a rotating stage.

Choose the model and the technique in the panel. In the simulation, the stage turns by itself.

50×: a mosaic of crystals

In plane-polarised light almost everything is colourless: quartz and feldspars. The brown mineral is biotite. Switch to crossed polars: the background turns black and each crystal lights up in grey, white or vivid colours, depending on the mineral and how it is oriented.

100×: the micas

Biotite goes from light brown to dark brown as the stage turns: this is pleochroism. It is darkest when its cleavage traces lie parallel to the polariser. Muscovite is colourless in plane-polarised light and, under crossed polars, shows vivid second-order colours: its birefringence is more than four times that of quartz.

200×: quartz and K-feldspar

Quartz is clear and, under crossed polars, grey or white: its birefringence of 0.009 times a thickness of 30 µm gives at most about 270 nm of retardation, first order. It often goes extinct in waves rather than all at once. K-feldspar (microcline) looks cloudy because of alteration and, under crossed polars, shows cross-hatched (tartan) twinning.

400×: twinning in plagioclase

Plagioclase is made of parallel lamellae whose crystal lattice is rotated from one to the next: polysynthetic twinning. As the stage turns, the lamellae go extinct in turn and light and dark bands appear. Every birefringent crystal goes extinct four times per turn, every 90°.

Which polarising microscope do you need?

For thin sections you need transmitted light with a polariser and an analyser, and a rotating stage. Compare the objectives and the polarisation accessories included in each spec sheet.