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The beauty of rocks under the microscope Québec Science

Obviously, the minerals do not really have such vivid colors as those which brighten up this report. However, these images are perfectly faithful to what can be seen under a microscope. The key is between the light source and the eye in the eyepiece: the polarizers (which filter light according to its direction) and the compensator (another thin strip of a mineral chosen for its properties). “These are not colored filters,” warns the researcher. Basically, each of these elements causes the direction of light to diverge, which the crystals in the sample also do due to their geometry, like a prism. The colors are therefore the sum of these influences.

Thus, we can easily distinguish biotite crystals in the rock: a color change occurs when the sample is rotated under a single polarizing filter. Then, when two filters are crossed, this mineral becomes practically opaque, therefore black, when the cracks which cross it are placed horizontally or vertically. A compensator is not necessary to recognize biotite, but in the case of apatite and feldspar it facilitates the task of identification.

The importance of the gaze

These techniques are centuries old. Other advanced tools are also used, such as scanning electron microscopy or techniques based on synchrotron radiation. As assistant editor at Journal of Metamorphic Geology, Professor Cesare is able to observe that “it has become tempting to skip the optical microscopy stage, to go directly to more sophisticated tools. But we then find ourselves building our reflections on fine analyzes without having a good idea of ​​how our rock is made. Without this larger vision, we can miss important things. “

It was precisely by observing garnets under a polarizing microscope that he discovered that they did not all have a cubic crystalline structure. However, they were until then considered as the cubic mineral par excellence! “Under the effect of two polarizers, a cubic mineral always appears black. But I realized that some garnets are not completely black. We continued the research and realized that several garnets are tetragonal ”, which was the subject of a publication in 2019 in the scholarly journal Scientific Reports. He is now trying to understand why the crystallization of this mineral can take a different form.

His photographs decorate not only his living room, but also geology departments and private homes all over the world: the professor markets his works… those of nature, one should say!

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