Sunday, December 9, 2007

Slice it Thin

The purpose of making thin sections is to create a thin, polished slice of rock 30 microns thick attached to a microscope slide that can be examined under a polarizing microscope to identify the mineral composition of the rock.  

The process starts by attaching the rock sample to a glass microscope slide.  Below the rock is being attached against the frosted glass side of microscope slides with epoxy on the pressure jig that provides even pressure and a consistent thickness of epoxy.  If samples are very porous or crumbly, they are first saturated with epoxy, sometimes using vacuum impregnation.  There can't be any air bubbles or unevenness between the rock and the slide surface.  It takes a couple of hours for the epoxy to cure on the hotplate where the jig rests.

The next step is to cut the rock samples down as soon as possible after the epoxy sets, especially large samples as they cool more unevenly and could generate enough force to crack the glass slides.  The excess sample is trimmed neatly off with a vacuum swing arm on a rock cutting saw (below).  The glass slides provide a good seal on the vacuum plate, the arm allows the new cut to be parallel to the surface of the slide so the finished product will be exactly the same thickness all over.

Next the slides with thicker-than-final rocks firmly attached are placed on the lapping machine to be slowly polished down to the final exactly-30-micron thickness.  A vacuum system again holds the smooth glass slide against the weighted chuck, holding the rock sample down to the grinding surface.  The grinding plate has to be absolutely flat or the finished slides will be uneven so Steve constantly monitors the process, moving slides and arms around.  In the picture below you can see one vacuum chuck loaded up with slides, ready to go on the lapping disc seen behind.  Various sizes of abrasive grit slurrys are used to provide the grinding action, getting finer and finer as the process comes to a close.  Using a fine grade grit at the end is important for optical quality work.  The final few microns are polished away by hand.
Steve Petrushak is an artist and miracle worker.  Samples so crumbly that we could barely (or couldn't)
 get them out of the core in one piece, he manages to create perfect thin section slides.  The ANDRILL team is lucky to have him!  He examines each finished slide to make sure it is ready to be used to identify the core's mineral components at that sampled depth.  He has requests for 600 thin section slides this season, which works out to about 50 every two days.  He is currently managing around 15 every day so will be continuing work after some of the team has left the ice.  To work in this field he says is useful to have some background in mineralogy, optical geology, and crystallography, as well as a good mechanical feel. 

The finished product!

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