Minerals Under the Microscope
Optical Mineralogy
Sep 23, 2009 Alexandra Matiella Novak
The science of studying minerals, or mineralogy, is one of the core principles of geology. It is usually one of the first introductory courses that geologists take in their studies and lays the foundation for being able to identify different types of rocks and, ultimately, the environment in which these rocks formed or were deposited.
Optical Mineralogy
Many minerals in rocks are identifiable by the naked eye, especially if the rock formed in an environment where mineral fluids cooled slowly, allowing for larger mineral growth. However, very often geologists find minerals that are too small or have been recycled, such as in sedimentary and metamorphic rocks, so that they are no longer identifiable to the naked eye. In these types of situations, studying the minerals using a microscope, or optical mineralogy, is the most reliable and efficient way to classify the mineral and its texture.
Thin Sections of Rocks and Minerals
Optical mineralogy involves using a petrographic microscope to look at a very thinly cut section of a mineral or rock. These thin sections are made by sawing a very thin slice of rock, gluing it to a microscope slide, filing down the thin slice until it is transparent, but still retains all the features of the original slice, and then gluing a microscope slide to the the other side. The glue used in the microscope slide does not interfere with the optical properties of the thin section. Making a good thin section takes practice and the hardest part is making sure the thin slice of rock is not filed completely away, or is not left too thick so that light can not be transmitted through it. A good thin section is about one-thousandth of an inch (30 micrometers) thick.
Petrographic Microscope
Petrographic microscopes are unique from other microscopes in that they are fitted with specially designed lenses intended for crystallographic analysis. It has a rotating table and polarizing lenses so that minerals can be identified based on how they behave at different wavelengths. There are also slots in the table to allow the placement of different filters cut from biaxial minerals, such as a quartz wedge, half-wave mica plate and quarter-wave mica plate. These filters further the analysis of the optical properties of minerals.
The Optical Properties of Minerals
The optical behavior of a mineral under a petrographic microscope is related to the chemistry of the minerals, the bonds of the molecules that make up the mineral and how light is transmitted, reflected and absorbed by these materials. For this reason, every mineral has different optical characteristics, although minerals that are very similar in chemical make-up can be very hard to distinguish from one another. Thin sections under a microscope often look like abstract art, with lots of colors and shapes. One can tell if a mineral has cleavage, which would show up as lines within a mineral grain. If the chemistry of the mineral is so that it completely absorbs light, then the mineral would be opaque and black. The shapes of the grains are also very telling, for example, quartz grains tend to be rounded.
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Source:
Nesse, William D. Introduction to Optical Mineralogy. NY: Oxford University Press, 1991.
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