Ivor and Olive Lewis on wed 5 apr 06
Passages in Dana's Mineralogy (p 200, p 460) giving a description of =
Perthite, a structure found in some felspar system found in some samples =
of Nepheline Syenite. Similarity with Pearlite structures that found in =
some Carbon steels prompted me to look at the phase diagram.
In the system K2O-Na2O-Al2O3-SiO2 the join between Na-K Felspars and =
Na-K Nephelines has a temperature range from 1086 +/- 5 deg C down to =
1020 +/-5 deg C. The lower temperature is at a Eutectic Point with =
Potash Felspar. Given this information the notion of a Natural Eutectic =
seems to be a plausible explanation for using Nepheline Syenite to =
adjust the temperature of a glaze down a couple of cones.
I hope that explains my ideas to your satisfaction.