Talc-Schist 1 / Chloritoid.
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This page and the following present some
micrographs and spectra obtained on a thin section of a talc-schist with
garnets and chloritoid. These two pictures demonstrate clearly the
strong pleochroism of chloritoids whose color changes from yellow to
green and blue. The first spectrum graph has been recorded on a 30µ thin section, the second and third graphs are spectra obtained with a slightly thicker section for 3 different crystals with polarizer orientations 0° and 90°. The strong color change with polarizer direction is mainly due to the intervalence charge transfer band ( Fe2+ - Fe3+) at around 600 nm. The NIR spectrum of Chloritoid also exhibits a strong dependency with polarization direction. The bands located at 900 nm and 1240nm are crystal field transitions of Fe2+ ions replacing Mg2+ in two different sites of the Chloritoid crystal structure. The small band at 1600nm is a harmonic vibration of the OH groups. It is not further split at higher resolution as shown below. |
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The small transitions below 550 nm are probably produced by crystal field forbidden transitions of Fe3+ ions. |
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