Petrographic microscope

The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks.

However, a "simple polarizing" microscope is easily made by adding inexpensive polarizing filters to a standard biological microscope, often with one in a filter holder beneath the condenser, and a second inserted beneath the head or eyepiece.

A particular light pattern on the upper lens surface of the objectives is created as a conoscopic interference pattern (or interference figure) characteristic of uniaxial and biaxial minerals, and produced with convergent polarized light.

In addition to modifications of the microscope's optical system, petrographic microscopes allow for the insertion of specially-cut oriented filters of biaxial minerals (the quartz wedge, quarter-wave mica plate and half-wave mica plate), into the optical train between the polarizers to identify positive and negative birefringence, and in extreme cases, the mineral order when needed.

As early as 1808, the French physicist Étienne Louis Malus discovered the refraction and polarization of light.

Plain light with the first filter (above), crossed-polarized light with both filters (below) in a volcanic lithic fragment (sand grain). Scale box in millimeters.
Leica DMRX incident light microscope with mechanical stage and Swift F automated point counter for analysis of organic composition of coal and rock samples
Thin sections under a microscope.
Photomicrograph of a thin section of gabbro in cross-polarized light