Dear Joern,
Most alkali loss in probe analyis is not volatilization, but
counting loss due to Na diffusion away from the beam site. Given the
structural role of Al in silicate minerals and amorphous silicates,
it seems most unlikely that either of these phenomena can explain
your observations. Your Crd alteration products are probably highly
hydrated, clay-like substances, possibly amorphous. The beam striking
these sorts of delicate materials commonly causes severe damage,
resulting in sputtering (associated with rapid H2O loss). If the beam
cross section were progressively reduced by this sort of damage, this
would explain the fall in Al count rate - less material in the
excited area of the sample an material is removed by sputtering and
shrinkage.
Cheers,
JC
--
John D. Clemens
Professor of Geosciences,
Director, Centre for Earth and Environmental Science Research
Editor in Chief - Electronic Geosciences
http://link.springer.de/link/service/journals/10069/index.htm
School of Earth Sciences and Geography, CEESR, Kingston University,
Penrhyn Rd, Kingston-upon-Thames, Surrey, KT1 2EE, UK
phone: +44 (0)20 8547-7023 fax: +44 (0)20 8547-7497
e-mail: [log in to unmask]
personal web page:
http://www.king.ac.uk/esg/staff/pages/clemens.htm
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Our passions cannot alter the facts, only hide them from us.
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