Quantitative absorbance spectroscopy with unpolarized light: Part II. Experimental evaluation and development of a protocol for quantitative analysis of mineral IR spectra

I. Kovács, Jörg Hermann, Hugh St C O'Neill, John Fitz Gerald, Malcolm Sambridge, Gábor Horváth

Research output: Contribution to journalArticle

117 Citations (Scopus)

Abstract

The predictions of the theory of light propagation in weakly absorbing anisotropic minerals are tested against systematic measurements of the infrared absorbance spectra of calcite, olivine, and topaz oriented in both principal and random sections, using both polarized and unpolarized light. We show that if the linear polarized maximum absorbance is smaller than ∼0.3, or if the ratio of maximum and minimum absorbance is close to unity, then (1) the polarized maximum and minimum absorbances as well as the unpolarized absorbance are, to a good approximation, linearly proportional to thickness, regardless of the direction of the incident light; (2) the angular variation of polarized light absorption is indistinguishable from the theoretical predictions within the uncertainty of the measurements; (3) for any section the unpolarized absorbance is the mean of the polarized maximum and minimum absorbance; and (4) the average unpolarized absorbance of randomly oriented grains is one third of the Total Absorbance (defined as the sum of the three principal absorbances). Therefore, calibrations relating Total Absorbance to absorber concentration in minerals that have been developed from measurements with polarized light parallel to the principal axes may be applied to measurements with unpolarized light on a population of randomly oriented sections. We show that 10 such measurements are sufficient to achieve a petrologically useful accuracy. The method enables water concentrations in nominally anhydrous minerals to be determined from samples where the preparation of oriented specimens is not feasible, such as high-pressure experimental runs and fine-grained mantle xenoliths. The method may also be used for obtaining quantitative measurements on low-symmetry minerals.

Original languageEnglish
Pages (from-to)765-778
Number of pages14
JournalAmerican Mineralogist
Volume93
Issue number5-6
DOIs
Publication statusPublished - May 2008

Fingerprint

absorbance
quantitative analysis
Minerals
spectroscopy
minerals
Spectroscopy
evaluation
mineral
Chemical analysis
polarized light
Light polarization
Topaz
Light propagation
Calcium Carbonate
electromagnetic absorption
predictions
calcite
olivine
Light absorption
protocol

Keywords

  • Absorbance spectroscopy
  • Calcite
  • Infrared spectroscopy
  • Nominally anhydrous minerals
  • Olivine
  • Topaz
  • Unpolarized light

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

Cite this

Quantitative absorbance spectroscopy with unpolarized light : Part II. Experimental evaluation and development of a protocol for quantitative analysis of mineral IR spectra. / Kovács, I.; Hermann, Jörg; O'Neill, Hugh St C; Gerald, John Fitz; Sambridge, Malcolm; Horváth, Gábor.

In: American Mineralogist, Vol. 93, No. 5-6, 05.2008, p. 765-778.

Research output: Contribution to journalArticle

Kovács, I. ; Hermann, Jörg ; O'Neill, Hugh St C ; Gerald, John Fitz ; Sambridge, Malcolm ; Horváth, Gábor. / Quantitative absorbance spectroscopy with unpolarized light : Part II. Experimental evaluation and development of a protocol for quantitative analysis of mineral IR spectra. In: American Mineralogist. 2008 ; Vol. 93, No. 5-6. pp. 765-778.
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N2 - The predictions of the theory of light propagation in weakly absorbing anisotropic minerals are tested against systematic measurements of the infrared absorbance spectra of calcite, olivine, and topaz oriented in both principal and random sections, using both polarized and unpolarized light. We show that if the linear polarized maximum absorbance is smaller than ∼0.3, or if the ratio of maximum and minimum absorbance is close to unity, then (1) the polarized maximum and minimum absorbances as well as the unpolarized absorbance are, to a good approximation, linearly proportional to thickness, regardless of the direction of the incident light; (2) the angular variation of polarized light absorption is indistinguishable from the theoretical predictions within the uncertainty of the measurements; (3) for any section the unpolarized absorbance is the mean of the polarized maximum and minimum absorbance; and (4) the average unpolarized absorbance of randomly oriented grains is one third of the Total Absorbance (defined as the sum of the three principal absorbances). Therefore, calibrations relating Total Absorbance to absorber concentration in minerals that have been developed from measurements with polarized light parallel to the principal axes may be applied to measurements with unpolarized light on a population of randomly oriented sections. We show that 10 such measurements are sufficient to achieve a petrologically useful accuracy. The method enables water concentrations in nominally anhydrous minerals to be determined from samples where the preparation of oriented specimens is not feasible, such as high-pressure experimental runs and fine-grained mantle xenoliths. The method may also be used for obtaining quantitative measurements on low-symmetry minerals.

AB - The predictions of the theory of light propagation in weakly absorbing anisotropic minerals are tested against systematic measurements of the infrared absorbance spectra of calcite, olivine, and topaz oriented in both principal and random sections, using both polarized and unpolarized light. We show that if the linear polarized maximum absorbance is smaller than ∼0.3, or if the ratio of maximum and minimum absorbance is close to unity, then (1) the polarized maximum and minimum absorbances as well as the unpolarized absorbance are, to a good approximation, linearly proportional to thickness, regardless of the direction of the incident light; (2) the angular variation of polarized light absorption is indistinguishable from the theoretical predictions within the uncertainty of the measurements; (3) for any section the unpolarized absorbance is the mean of the polarized maximum and minimum absorbance; and (4) the average unpolarized absorbance of randomly oriented grains is one third of the Total Absorbance (defined as the sum of the three principal absorbances). Therefore, calibrations relating Total Absorbance to absorber concentration in minerals that have been developed from measurements with polarized light parallel to the principal axes may be applied to measurements with unpolarized light on a population of randomly oriented sections. We show that 10 such measurements are sufficient to achieve a petrologically useful accuracy. The method enables water concentrations in nominally anhydrous minerals to be determined from samples where the preparation of oriented specimens is not feasible, such as high-pressure experimental runs and fine-grained mantle xenoliths. The method may also be used for obtaining quantitative measurements on low-symmetry minerals.

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