An invaluable tool to the
petrologist (a
geologist who studies the composition of rocks), The petrographic microscope is a specialized type of
microscope that is used to analyse the
optical properties of rocks and
minerals. These properties can be used to: identify minerals, calculate precise
chemical compositions of individual minerals and observe
microtextures within the rock (used to determine formation and
metamorphic history).
Sedimentary petrologists and
paleontologists also use these instruments to examine and identify various macroscopic and microscopic
fossils, like
foraminifera,
ostracods and
crinoids.
The petrographic microscope was developed in Germany in the late 19th Century. As understanding of the the chemical structures and optical properties of minerals developed, so did the complexity and usefulness of the microscopes. Today, the two major manufacturers of petrographic microscopes are Zeiss and Nikon.
In order to examine a rock sample under a petrographic microscope, a thin section must first be produced. First, the rock sample is cut to size and polished smooth. A glass slide then is adhered to the sample using a clear epoxy. The rock is ground away -- sometimes by hand for cursory study, usually by a more precise, mechanical thin sectioner -- until the thickness is approximately 30 microns. Grinding the rock this thin permits light to pass through the sample, with the crystalline nature of the minerals causing refraction of this light. The light is then passed through a prism, providing the observer with a complete image.
Modern petrographic microscopes can be divided into three functional parts: the head, the stage and sub-stage assembly and the base:
The head comprises the eyepiece (may be single or binocular), a prism, two to five objectives (magnifying lenses, typically ranging in power from 2X to 40X) and several lenses and accessories which may be entered into or removed from the system to test the various optical properties of minerals, including the Bertrand lens, mica plate and gypsum plate. A camera (still or video) may be used in place of, or in conjunction with, the ocular eyepiece in order to take photomicrographs.
The stage is precisely that... a stage on which the slide rests. The stage can be rotated a full 360 degrees, permitting the thin section to be subjected to polarised light at all possible angles. The sub-stage assembly includes a condenser (usually used with the Bertrand lens), a diaphragm (used to limit the amount of light shone on the slide, useful to find relief in minerals) and the polarizer (whose principal function is to examine extinction in minerals). The stage may be locked and fitted with a scaling device in order to perform point counts -- a sampling method used to determine the bulk content of minerals in a rock sample.
The base contains the light source (a 15W light bulb), which is then reflected towards the stage and passed through a blue filter. Mounted on the base are the focus knobs, which focus the image by controlling the height of the stage. Usually the stage is mounted on rubber feet, to prevent the microscope from sliding around during use.
Some minerals cannot be analysed using normal petrographic microscopes, as they are opaque, even in thin section. The minerals (usually ore-bearing sulfides like chalcopyrite, galena or cerrusite) require a reflecting light microscope. A light source is shone on the thin section from above, and the light reflected from the sample is viewed. These instruments are much less complex, but less information can be collected from them.
Petrographic microscopes are expensive, averaging $10,000 per instrument. As they are required for any intermediate-level mineralogy or petrology course, most universities with geology or earth science programs own several of them.
Sources:
University of British Columbia, EOSC 221 -
http://www.science.ubc.ca/~geol202/labs/
Evolution of the Petrographic Microscope and its Impact on Advances in the Geological Sciences -
http://gsa.confex.com/gsa/2001AM/finalprogram/abstract_20271.htm
Experimental Design -
http://web.mit.edu/12.000/www/finalpresentation/experiments/section.html