Petrographic microscope

Apetrographic microscopeis a type ofoptical microscopeused to identifyrocksandmineralsinthin sections.The microscope is used inoptical mineralogyandpetrography,a branch ofpetrologywhich focuses on detailed descriptions of rocks. The method includes aspects ofpolarized light microscopy(PLM).

Plain light with the first filter (above), crossed-polarized light with both filters (below) in avolcanic lithic fragment(sand grain). Scale box in millimeters.

LeicaDMRX 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.

Photomicrographof a thin section ofgabbroin cross-polarized light

Description

Depending on the grade of observation required, petrographic microscopes are derived from conventionalbrightfield microscopesof similar basic capabilities by:

• Adding a Nicol prismpolarizerfilter to the light path beneath the sample slide
• Replacing the normal stage with a circular rotating stage (typically graduated withvernier scalesfor reading orientations to better than 1 degree of arc)
• Adding a second rotatable and removableNicol prismfilter, called the analyzer, to the light path between objective and eyepiece
• Adding aphase telescope,also known as a Bertrand lens, which allows the viewer to seeconoscopic interference patterns
• Adding a slot for insertion ofwave plates

Petrographic microscopes are constructed with optical parts that do not add unwanted polarizing effects due to strained glass, or polarization by reflection in prisms and mirrors. These special parts add to the cost and complexity of the microscope. 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. These can be sufficient for many non-quantitative purposes.

The two Nicol prisms (occasionally referred to asnicols) of the petrographic microscope have their polarizing planes oriented perpendicular to one another. When only anisotropicmaterial such as air, water, or glass exists between the filters, all light is blocked, but most crystalline materials and minerals change the polarizing light directions, allowing some of the altered light to pass through the analyzer to the viewer. Using one polarizer makes it possible to view the slide in plane polarized light; using two allows for analysis under cross polarized light. A particular light pattern on the upper lens surface of the objectives is created as aconoscopic interference pattern(or interference figure) characteristic of uniaxial and biaxial minerals, and produced withconvergent polarized light.To observe the interference figure, true petrographic microscopes usually include an accessory called a Bertrand lens, which focuses and enlarges the figure. It is also possible to remove an eyepiece lens to make a direct observation of the objective lens surface.

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 plateandhalf-wave mica plate), into the optical train between the polarizers to identify positive and negativebirefringence,and in extreme cases, the mineralorderwhen needed.

History

As early as 1808, the French physicistÉtienne Louis Malusdiscovered the refraction and polarization of light.William Nicolinvented a prism for polarization in 1829, which was an indispensable part of the polarizing microscope for over 100 years. Later the Nicol prisms were replaced by cheaperpolarizing filters.

The first complete polarizing microscope was built byGiovanni Battista Amiciin 1830.

Rudolf Fuess built the first polarization microscope specifically for petrographic purposes in 1875. This was described byHarry Rosenbuschin the yearbook for mineralogy.