Day #116 Deskcrop: Half Dome Granodiorite (Thin Section)

I’m kicking this week of thin section deskcrops with a rock you’ll already be quite familiar with if you’ve been following my blog this year – that’s right, it’s another look at the Half Dome Granodiorite! We’ve previously seen it pulverized, in deskcrop, and outcrop. Today’s views are of thin sections (30 micrometer thick slices of the rock) as seen under a petrographic microscope (or technically, a petrographic macroscope, in this case).

The plane light view is nothing spectacular, since the minerals that make up the bulk of this rock (quartz and plagioclase) are low relief, uncolored minerals in plane light. Near the center of the plane light view there is a grain of biotite with secondary chlorite oriented parallel to cleavage in a NE-SW orientation in this grain. Just to the right of it is a large green crystal of hornblende. Both the biotite and hornblende are pleochroic (they change colors as the stage is rotated in plane light), but I’ve only photographed this orientation.

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Half Dome Granodiorite (plane light)

In cross polarized light the plagioclase grains really steal the show. There are prominent grains that show both Carlsbad and albite twinning, as well as a couple that show concentric compositional zoning (check out the NE quadrant). To get a closer look at any of these photomicrographs click through to the image on Flickr and then click the link to “All Sizes” above the photo there.

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Half Dome Granodiorite (X-polarized light)

Explore this and all of my 2010 Deskcrops and Outcrops in Google Earth!