Date: Tue, 13 Sep 1994 16:07:22 -0400 (EDT)
Subject: Re: biotite analysis problem
From: Gregory H Symmes
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Hi John-
I generally use end-member, anhydrous silicate standards for biotite analyses
and get good results (basically the same approach that we used at Hopkins, with
Si, Al, Ca on anorthite, Fe on fayalite, Mg on enstatite, K on microcline, Na
on albite, Mn on rhodonite, Cr on Chromite, Ti on ilmenite, etc. I generally
do not measure O directly, but one of our grad students has used O analyses
pretty successfully on some of his experimental biotites (he usually got low
totals, but they were due to fine-grain size of synthetic crystals. We
sometimes use one of the well-characterized biotites as a working standard
(rather than a standard) and find that we can reproduce the known compositions
quite well.
I tend to trust end-member mineral compositions more than complex solid
solutions, so I generally avoid using natural minerals as standards. It might
be worth trying a general, 10-element analysis package with different
standards to check whether the problem is caused by the biotite standards that
you are using. The other real possibility, is that his biotites are altered to
some degree to chlorite. Are K totals very low? I found that "good-looking"
biotites can actually be intergrowths of chlorite+biotite at TEM scale. In my
rocks, I will sometimes use K intensity to search for unaltered biotites. (You
can peak up a spectrometer to K and use the audio to search for high K regions
on biotite). As far as the biotite+chlorite intergrowths, Veblin and Ferry
wrote a paper in 1983? on TEM microstructures of intergrowths. Good luck.