Window Into A Microscopic World

Thin sections of rock only 0.03 millimeter or 0.00118110236 of an inchthick on a glass slide offer us incomparable insight into the geological history of our Earth.

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Shearing textures in a garnet schist gneiss – crossed polarized view

As a geology student several years ago I spent hundreds of hours making and looking at hundreds of thin sections (for more details check out the third paragraph in this post). I was mesmerized by these slender slivers of rock that offer a window into a geologically microscopic world. There was always something new to see, something to learn, and so much to marvel at.

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Shearing textures in a garnet schist gneiss – plane polarized view

Metamorphic rocks are rocks that have been changed without melting from one type of rock to another under the influence of heat and/or pressure. In school I was totally consumed by the metamorphics in the Beaver Dam Mountains of extreme southwestern Utah.  They still maintain a profound hold on my psyche, even though I am no longer a student.

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Sheared gneiss with dark gray garnet being replaced by brown biotite – crossed polarized view

 

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Sheared gneiss with dark gray garnet being replaced by brown biotite – plane polarized view

Petrographic microscopes allow us to study the optical properties of minerals. These mineral properties can offer intriguing clues to a rock’s tectonic history involving shearing, uplift, and decompression from pressures and temperatures deep within the Earth.

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Mark C., captivated by his thin sections

Can you discern a sense of shearing, a flowing movement in these ancient minerals?

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Sheared gneiss mylonite, mainly quartz and biotite – crossed polarized view

 

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Sheared gneiss mylonite, mainly quartz and biotite – plane polarized view

 

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Shearing in black garnets and thready multicolored sillimanite – crossed polarized view

 

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Shearing in garnets and thready sillimanite – plane polarized view

And so from this

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Sillimanite breaking apart within a schist – crossed polarized view

…and this…

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Sillimanite breaking apart within a schist – plane polarized view

…and this…

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Sheared plagioclase breaking apart in a schist-gneiss mylonite – crossed polarized view

…and this…

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Sheared plagioclase breaking apart in a schist-gneiss mylonite – plane polarized view

…we can one day learn how this came to be.

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Beaver Dam Mountains, southwestern Utah