Importance And Use Of Petrographic Microscopes

Petrographic microscopes, also known as polarizing microscopes, are specialized instruments used widely in the study of the optical properties of minerals and rocks under polarized light. They are used for the identification of rocks and minerals; arrive at their chemical compositions and identify micro-textures in a rock. Petralogists identify rock fossils with the help of these type of microscopes.

Polarizing microscopes were first developed in Germany in the later part of the 19th century. Their usefulness has increased manifold over time, and thus they have become much advanced as compared to their earlier counterparts. The latest models are quite expensive, and come for around $10,000 per piece. The technology used for the polarization of these microscopes leads to the high cost of production. Getting the perfect polarized lenses mounted adds further to the cost. 

Petrographic microscopes come in two varieties- 

1. Those that have the feature of transmitted polarized light only.
2. Those that have, in addition to the above-mentioned feature, vertically incident reflected polarized light through the objective lens. An instrument containing both types of light is known as an ore microscope. These have the property to examine solid, polished rock specimens. Here, the polarized light passes through another polarizing light filter known as an analyzer.

Common applications of polarized microscope includes mineral and rock classification, bio-medical research, medical applications including study of microscopic crystals in urine, meteorite thin sample study, use in the petroleum sector and in geological engineering. 

Structure Of A Petrographic Microscope 

Latest petrographic microscopes can be segregated into three well-designed parts –
i. the head.
ii. the stage and sub-stage assembly.
iii. the base.

The substage condenser (used with Bertrand lens) can be centered as well as focused, and it contains field and aperture diaphragms. The polarizer (used to determine mineral extinction) comprises of the upper/lower polarizing devices and an accessory lens for convergent light. It is possible to rotate the stage to a complete 360 degrees, thus allowing the thin section to be exposed to the polarized light at all angles.

The head consists of the eyepiece, magnifying lenses, and a prism. A still or video camera may be used in place of the eyepiece, so as to take photomicrographs. 

The base of this microscope houses the light source that gets reflected towards the stage, and subsequently passes through a blue colored filter. 

How To Study A Sample Under A Polarized Microscope 

A thin section of the sample must be recovered in order to study it under a polarized microscope. The first stage involves the cutting of the sample to size and then polishing one side smooth with a fine abrasive such as diamond powder. Next, a glass slide is used to mount the sample, and the other side is ground by a mechanical cutter to a thickness of about 30 microns, and covered by a thin cover glass. This allows light to pass through the sample and results in normal incidence. Due to the crystalline property of minerals, the light also gets refracted. In the final stage, a prism is used for the ray of light to pass through, thus giving the observer a completely lucid image. 

A normal petrographic instrument is unable to study certain minerals as they turn out to be opaque even in very thin sections. Examples of such minerals include ore-bearing sulfides eg. galena, chalcopyrite. In such cases, reflecting light microscope may be used where an exterior light source from above is shown on the sample. As the light gets reflected, studies are carried out subsequently. 

Mineralogy, geology and pictography students benefit the most from a pectographic microscope. It is quite uncommon to use such a microscope at home, but a hobbyist or a mineral collector might purchase one to examine rocks under this.