HR: 18:05h
AN: MR44A-08    [Abstracts]
TI: QUANTITAIVE IR SPECTROSCOPY WITH UNPOLARIZED LIGHT
AU: * Kovacs, I
EM: istvan.kovacs@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, 61 Blg, Mills Road, Canberra, ACT 2614, Australia
AU: Hermann, J
EM:
AF: Research School of Earth Sciences, Australian National University, 61 Blg, Mills Road, Canberra, ACT 2614, Australia
AU: O`Neill, H S
EM:
AF: Research School of Earth Sciences, Australian National University, 61 Blg, Mills Road, Canberra, ACT 2614, Australia
AU: Sambridge, M
EM:
AF: Research School of Earth Sciences, Australian National University, 61 Blg, Mills Road, Canberra, ACT 2614, Australia
AU: FitzGerald, J
EM:
AF: Research School of Earth Sciences, Australian National University, 61 Blg, Mills Road, Canberra, ACT 2614, Australia
AU: Horvath, G
EM:
AF: Institute of Mathematics, Eotvos University, Pazmany Peter setany 1/C, Budapest, 1117, Hungary
AB: A theory on light propagation in weakly absorbing anisotropic minerals is presented by 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 maximum principal linear polarized absorbance is smaller than 0.3 or the ratio of maximum and minimum absorbance is close to unity than: (1) the polarized maximum and minimum absorbances as well as the unpolarized absorbance are linearly proportional to thickness regardless of the direction of the incident light; (2) the measured angular variation of polarized light absorption as a function of maximum and minimum values is indistinguishable from the theoretical predictions [A(θ) = A_maxcos²(θ)+A_minsin²(θ)] within the uncertainty of the measurements; (3) for any section the unpolarized absorbance is the mean of the maximum and minimum polarized absorbance; (4) the average unpolarized absorbance of randomly oriented grains is exactly one third of the total absorbance, which is the sum of the principal polarized absorbances. Therefore, calibrations relating total absorbance to water concentration in minerals that have been developed from measurements with polarized light parallel to the principal axes may be used with unpolarized light on a population of randomly oriented sections, provided that enough measurements can be obtained to constrain the average. We show that ten such measurements are sufficient to achieve a petrologically useful accuracy. This method allows the water concentrations in nominally anhydrous minerals from high-pressure experimental runs and fine-grained mantle xenoliths to be measured, for which the preparation of oriented samples of such specimens is usually not feasible.
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3630 Experimental mineralogy and petrology
DE: 3694 Instruments and techniques
SC: Mineral and Rock Physics [MR]
MN: 2007 Joint Assembly