An elevated temperature X-ray study of synthetic disordered Na-K alkali feldspars |
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Authors: | C M B Henderson |
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Institution: | (1) Department of Geology, The University, M13 9PL Manchester, England |
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Abstract: | The temperature dependence of cell parameters for three disordered, synthetic alkali feldspars (Or19, Or38, and Or100) has been determined up to 1,000 °C. The samples show no change in composition or degree of Si-Al disorder during the experiments. The triclinic-monoclinic inversion in the sample of composition Or19 occurs at 560 °±10 °C and is accompanied by changes in the rates of expansion of a, b and c; the rate for a increases and those for b and c decrease above the inversion. The b and c parameters in Or100 show small decreases with increasing temperature and this may be due to thermal motion effects causing a contraction of cell directions that are fully expanded at room temperature. Calculation of the thermal expansion ellipsoids for the monoclinic phases shows that the major expansion coefficients ( 1) for all three samples are more than an order of magnitude greater than the intermediate ( 2) and minor ( 3) coefficients. Thus the thermal expansion of these phases is dominated by that of 1 which makes an angle of 22 ± 4 ° with+a; this orientation is parallel to that of the short M-OA2 bonds. The thermal expansion mechanism for monoclinic, disordered alkali feldspars may involve tilting within the framework releasing compression along this direction and allowing the M-OA2 bonds to show high expansion rates. The stretching of the crankshaft units, which are parallel to a, may only play a subordinate role in controlling the expansion of the feldspar framework. |
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