Quantitative Spectroscopic studies.
Spectroscopic methods remain the most important way to establish the presence
of hydrous components in minerals and to differentiate water from hydroxide
ions. They are sensitive, rapidly applied and convenient to conduct, but
are not intrinsically self-calibrating. To address the problem of quantitative
calibration, we are attempting to develop independent absolute calibrations
for the spectroscopic methods.
The Hydrogrossular Calibraton
When there are 'large' concentrations of OH in minerals, the independent
analysis of the hydrogen content can be reliably conducted by a variety
of methods. Hydrogrossular can be found in nature with a variety
of different H2O contents and can be synthesized with essentially
every atom of Si replaced
by four H+ ions. A calibration
of the IR spectrum of OH in grossular can be readily assembled from a combination
of natural and synthetic samples.
Absolute Hydrogen Manometry
Absolute hydrogen manometry involves extraction of the hydrogen from a
sample under high vacuum and temperatures and collection and separation
of the evolved gasses. The hydrogen-bearing component converted to molecular
hydrogen and measured in gas manometer. This method is widely used
in isotopic studies, but is not always suitable for calibration of the
infrared studies of 'nominally anhydrous' minerals. The primary challenges
come from the fact that most minerals of interest contain either fluid
inclusions or cracks and cleavages decorated with fine-grained hydrous
minerals. Consequently, only inclusion-free portions of samples can
be used for analysis of the low concentrations found in the 'nominally
anhydrous' minerals. Furthermore, sizable
quantities (grams) of such high-quality material is usually required
for these analyses. In spite of these difficulties, a selections
of minerals have been successfully analyzed by this method and form the
basis of some of our absolute
calibrations.
Nuclear Reaction Analysis
We are collaboration with the research group of Prof. Freidel
Rauch at the University of Frankfurt, Germany, who conducts nuclear
reaction analyses for the determination of hydrogen. The nuclear methods,
while not routine, require only a single calibration after which they can
be used on any material without the need for a mineral-specific calibration.
Rauch's group has made much progress in bringing these methods into the
realm of quantitative ppm analysis. Calibrations
of adequate quality are now emerging from this method for minerals with
10's to 100's of ppm H2O.
Correlations involving spectroscopic data
Former postdoc Eugen Libowitzky now
at the Institut
für Mineralogie und Kristallographie , University of Vienna, Austria,
examined the intensity of the OH bands in minerals in systems with different
degrees of hydrogen bonding. Previously, trends relating the intensity
of OH absorption to the length of the O···H–O bond
were noted in the study of the spectra of chemical compounds. Libowitzky
studied minerals with extremely short hydrogen bonds (such as pectolite
and mozartite) and established that similar
quantitative trends are observed in the spectra of minerals containing
stoichiometric amounts of OH. When trends such this can be established
for a group or class of minerals, they allow infrared spectroscopy to be
used for absolute H determination in minerals without the necessity of
the mineral-specific calibrations discussed above.
Much of our current effort is directed at addressing the apparent deviation
of the calibrations for minerals with low levels of OH f rom the general
calibration trends obtained with minerals with high concentrations of hydrogen.
Details of quantitative spectroscopic analysis
A variety of factors influence the accuracy of infrared spectroscopic measurements.
One of the more important is the quality of the polarizers used to examine anisotropic solids. We have examined
the effect of the polarizers and have shown that poor polarization efficiency
is the most obvious potential source of large error in the quantitative
measurement of a spectrum of an anisotropic crystalline solid.