High potassic feldspar

High Potassic Feldspars. A high potassic feldspar from Kingston, South Australia, (mole % K Na Ca - 81 18 1) represents a typical NTL spectrum for members of this group. Figures 2a and 3a show the 3-D and contour plots respectively. An emission band near 400 nm with FWHM of about 40 nm is prominent in the glow-curve. The NTL peaks occur between 250-350°C followed by another high temperature peak around 500°C. 

The Sodic End Members. Figures 2b and 3b show the NTL behaviour of an albite from Amelia, Virginia, U.S.A. which has very low K and Ca content (K Na Ca - 2 98 -). The main emission band is around 560 nm which peaks near 320°C. This broad (in wavelength and temperature) peak is characteristic of low K, low Ca feldspars. There is also a low intensity emission at shorter wavelengths and higher temperatures. The Amelia albite emits two to three times more TL light per mg than the high potassic end-member feldspars and considerably more than those of intermediate composition. 

Potassic feldspar is particularly appreciated by ceramists because its reaction with siUca leads to the formation of a liquid whose relatively high viscosity decreases slightly when the temperature increases. This behavior is considered as a guarantee against the excessive deformation of the pieces during the heat treatment. 

The studies cited above deal with deeply buried sediments which occur in areas of low geothermal gradients. Further the rocks are basically sodi-potassic, at least in the silicate aggregates, and most often contain a potassic phase such as illite, feldspar or zeolites. In instances where these chemical conditions prevail and where the geothermal gradient is high (Muffler and White, 1969 Browne and Ellis, 1970) the same temperature-mineralogy relations seem to hold, 100-120°C appears to be the upper limit of fully expandable montmorillonite stability. However, R. 0. 

---