Sedimentary rocks are made of grains compressed together and cemented by natural cement.
Metamorphic rocks are made of crystals and form when existing rocks are changed by increasing temperatures and pressures.
Thin section under plain polars
Light can pass through rock slices of this thickness so that the details can be seen under a microscope. The microscopes that geologists use to study thin sections of rock include two polarizing filters. The lower polar is normally used and thin sections are viewed in ‘plane-polarized light’ (light waves on one plane only). In plane-polarised light, minerals exhibit their ‘true’ colour.
Thin section under crossed polars
The polarizing directions of the two filters are at right angles to each other. The upper polar (also known as an analyser) can be moved in and out of the path of light. When the upper polar is introduced into the path of light, only minerals which are ‘optically active’ allow light to pass. The colour that is observed is an ‘interference colour’; and not true colour. This is known as viewing the thin section under ‘crossed polars’.