Quantitative absorbance spectroscopy with unpolarized light

 

István Kovács 1 , Jörg Hermann 1 , Hugh St. C. O'Neill 1 , Gábor Horv áth 2 , John FitzGerald 1 , Malcolm Sambridge 1
1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Institute of Matemathics, E ötvös University , Budapest , 1117, Hungary

 

We tested the predictions of the absorbance theory of light propagation in anisotropic minerals with systematic measurements of calcite and olivine in oriented and random sections through the minerals using polarized and unpolarized light. Five principal predictions of the absorbance theory have been tested: 1) We show that polarized maximum and minimum absorbance as well as unpolarized absorbance is linearly scaled with thickness regardless of the direction of the incident light. 2) It is demonstrated that the measured angular variation of polarized light as a function of maximum and minimum values is within error indistinguishable from the theoretical predictions. 3) It has been confirmed that unpolarized absorbance is always the mathematical average of the maximum and minimum polarized absorbance in any section. 4) The unpolarized absorbance for different arbitrary sections has been successfully calculated with olivine grains, for which the orientations have been determined by Electron Backscatter Diffraction. 5) The measured average unpolarized absorbance of randomly oriented grains is exactly one third of the Total Absorbance, in agreement with the absorbance theory. Because of this latter postulate, previous calibrations relating Total Absorbance to water concentration in minerals, which were developed for polarized light in principal sections, may be used in conjunction with unpolarized measurements according to:

where A i is the integrated unpolarized absorbance of individual analysis, n is the number of analyses and k pol is the calibration factor for polarized light. This method allows calculating water concentrations in nominally anhydrous minerals from high-pressure experimental runs and fine grained mantle xenoliths, specimens in which preparation of oriented samples is usually not feasible.

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