Monday, February 6, 2012

Window Into A Microscopic World

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

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. 

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. 

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

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. 

Mark C., captivated by his thin sections

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

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

Sheared gneiss mylonite, mainly quartz and biotite - plane polarized view
Shearing in black garnets and thready multicolored sillimanite - crossed polarized view

Shearing in garnets and thready sillimanite - plane polarized view
And so from this

Sillimanite breaking apart within a schist - crossed polarized view

…and this…

Sillimanite breaking apart within a schist - plane polarized view
…and this… 

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

…and this…

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

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

123_2318Beaver DamMountains
Beaver Dam Mountains, southwestern Utah


  1. These images are Amazing. How fun to look at rock from this perspective. How did you get the photos?

    1. Gaelyn - There is a camera attached to the microscope. You insert a camera card into a slot, focus, click a button, and viola! Above the eyepiece is a small screen where you can see what your chosen image will look like.

      I had to put the camera card back in my camera and download the images to my computer that way (could not insert the card directly into my specific computer).

  2. How do you cut the rock into such thin pieces? Yikes!

    1. Marianne - You cut the piece to about fist size on a rock saw (if necessary), then use a smaller saw to cut and shape the rock so it's dimensions are about 1 inch long x 1/2 inch wide x 1/4 inch thick. Then, one side is polished by hand with a really fine watery grit, dried, and then glued onto the slide. When this glue is dry a third piece of equipment, a smaller grinder, is used to lop off as much of the remaining rock as possible. Finally, the thin section is ground down (We had to do it by hand but there are machines to do this) using the fine grit until it is the appropriate thickness to identify the minerals. The whole process can take several days to complete.

      I absolutely loved making and looking at thin sections when I was in school! Truly a window into another world.