Eye to the Microscope

On the next microscope over, Dr. Kari Bassett spends much of her time examining and describing thin section slides like the one shown twice in different lights below (tune in tomorrow for all the details on how these thin slices of rock are made). Even though we look at them both under a microscope with different lights to learn about them, one major difference between the thin section slides and the smear slides I am working with is that the thin section is a slice of rock of known thickness - 30 microns.  Knowing the thickness of the slice allows scientists to use light accurately identify the minerals that make up the rock. A "rock" is anything made of multiple minerals. 

On the first look at a slide, the aim is to categorize the compositional type of the rock the slice was taken from (e.g. mostly terriginous) and get some grain size data.

When Kari looked at the slide above (plane polarized light on the left, cross polarized light on the right) some of the things she noticed were the two large chunks of material included in this rock.  In plain polar the one on the left is transparent and yellow, in cross polar it is black and opaque (can't see through it) - this means it is composed of glass.  How on earth does glass get into rock?  Volcanoes!  This whole area is tectonically and volcanically active.  We even have an active volcano, Erebus, on the island we are living on.  The bubbles in the glass indicate that the eruption occurred in cold conditions, the frothy lava didn't off-gas entirely before solidifying. Since it is only a small piece of volcanic glass, it has travelled to the place where it was incorporated into the sea floor.  The chunk on the right side of the slide, opaque and black in both types of light, was changed by the weathering process into pyrite or magnetite.  Overall this is an iron rich sample.

Even if you don't have one of the vital handbooks for checking on what you are seeing under the microscope, like The Colour Atlas of Rocks and Minerals in Thin Section or Microscopic Identification of Minerals, take a look at the paired images of a thin section slide below and describe features you notice.

Of the many things you might notice, look at the two grains that are brownish tan in the plain polar light on the left.  These demonstrate why mineral identification of grains needs both plain polar and cross polar examination.  When you look at the same two grains under cross polar, one is still brown and transparent, the other has turned black!  That one is glass, the other pyroxene (also a volcanic mineral).  Like the pyroxene in this slide, some minerals look the same under either light, others change a lot.  Either way, it tells us about what type of mineral it is.

What types of things can you think of that you need different sources of information about to be sure you are accurate?  What might be a good analogy for thin section mineral identification? 

For more on thin section identification, check out this online guide.