Elizabeth A. Johnson and George R. Rossman
Division of Geological and Planetary Sciences, California Institute of Technology
Pasadena, CA 91125-2500, USA
American Mineralogist 88, 901-911.
A universal absorption coefficient was determined for quantitative analysis
of OH and H2O in feldspars using infrared spectroscopy. 1H
MAS (magic-angle spinning) NMR spectroscopy was used to determine the H concentration
in three alkali feldspars, and for the first time, eight plagioclase samples.
To accurately measure structural H concentration in samples with low H (<1000
ppm H2O) it was necessary to eliminate the signal due to adsorbed
water in the powdered NMR sample. The pegmatitic and metamorphic albite samples
are transparent, but contain variable (40–280 ppm H2O) concentrations
of microscopic to sub-microscopic fluid inclusions. The pegmatitic albites
also have sharp bands in the mid-IR similar to the OH bands found in quartz.
The other plagioclase samples used in the IR calibration have broad anisotropic
bands around 3200 cm–1 in the mid-IR and weak combination stretch-bend
bands near 4550 cm–1 in the near-IR, indicative of structural
OH. The OH vector in plagioclase is preferentially aligned parallel to the
crystallographic a axis. The concentration of structural OH in the plagioclase
samples ranges from 210–510 ppm H2O by weight. The microcline
samples contain structural H2O molecules (1000–1400 ppm H2O)
and the sanidine sample contains structural OH (170 ppm H2O).
An approximately linear trend is produced when the total integrated mid-IR
absorbance is plotted vs. the concentration of structural H determined from
NMR (OH and H2O) for plagioclase and alkali feldspars. The integral
absorption coefficient for the total mid-IR peak area is 15.3 ± 0.7
ppm–1·cm–1 [107000 ± 5000 L/(mol H2O·cm2)]
for natural feldspar samples that contain structural OH or H2O.
Measurements of band areas of unpolarized IR spectra on (001) cleavage fragments
provide an estimate of H concentration for alkali feldspars, but this method
does not work for most plagioclase samples.