Sunday, November 6, 2011

Grains of Sand

About a month ago I told you about my piles of sand.  Now I am headed for single grains of sand and you are invited to follow along with my next science installment!

My last science installment followed the steps involved in generating several vials of sand with magnetic properties close to those expected for my mineral of interest- garnet.  This process narrows the search field, but unfortunately never yields a vial of pure garnet.  That's where the next step comes in.

The next step is known as hand-picking.  It takes place under a stereoscope, which looks like this:
Stereoscope on and ready to go
Basically, it's a microscope that allows the user to examine three-dimensional samples under varying magnifications and light intensities.  This particular scope is also outfitted with a digital camera for taking pictures of the individual sand grains when necessary.

The samples, which can be seen in the vials to the right of the scope above, are dumped into a weighing boat (the blue plastic dish the vials are in- above) or a clean glass dish.  The dish and sample are placed under the stereoscope.  Then I have to use the magnifying powers of the stereoscope, very fine tweezers, and my mineral identification skills to sort the garnet grains from all of the other grains of sand, proceeding one grain at a time.

PTFE-coated Tweezers, ink pen and lab notes for scale
I have two pairs of tweezers- one made from non-magnetic stainless steel and the other pair coated in PTFE (Teflon).  Both are considered needle-point and are essentially some of the finest tipped tweezers available.  They have to be...I am using them to pick up individual sand grains!  Why two pairs you ask?  The pair coated in Teflon is intended only for use with my detrital garnet grains, to minimize the exposure of the grains to unnecessary metal.  Contact with metal objects could, theoretically, contaminate my samples and skew my results since I am working with such extremely small sample volumes.  I am pretty sure that any potential impact of metal contact will be mitigated by later processing, but it never hurts to avoid the potential problem altogether when possible!  

Ultimately, each grain of garnet is individually picked out of my piles of sand using the tweezers and placed in a new vial containing only garnet from a single sample.  Generally, garnet is fairly easy to identify beneath the stereoscope's magnification, since it is pink to red to reddish brown.  Most of the other minerals that end up with it when grouped magnetically are nowhere near this color spectrum.

Hand-picking continues until I have gathered a vial with enough garnet to be analyzed.  This can take quite a bit of time, since each grain is selected individually.  Once enough grains are selected, it's off to the clean lab!  More on those adventures will be coming soon..... 

Of course, this is nowhere near an ideal system for mineral separation yet.  My original sand contains an extremely large volume of two minerals known as quartz and feldspar.  They are so abundant, that although magnetic separation should keep them far away from the garnet, they have permeated everything and to make matters worse, the feldspar is pink!  I am currently researching additional processing steps to add to our current mineral separation methods, such as heavy liquids and/or a water table, to eliminate this problem. Hopefully there will be posts in the future about new processes and procedures that I have added to our lab's capabilities to deal with these issues.


  1. I think it's awesome you have the patience to pick through sand grains because I know I couldn't do it. I look forward to reading more about your research!

  2. Random thought from a chemist...I was wondering what your method for magnetic separation was. Is there a way that gravity is blurring your gradient, and would changing the orientation of the field help out?

    As crazy as it sounds, I'm actually missing the lab a bit when I read your blog. Keep up the good work!