An elevated temperature X-ray study of synthetic disordered Na-K alkali feldspars

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 (α1) for all three samples are more than an order of magnitude greater than the intermediate (α2) and minor (α3) coefficients. Thus the thermal expansion of these phases is dominated by that of α1 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.