La dilatation thermique des feldspaths

Forelesninger holdt ved Advanced Study Institute of Feldspar - Oslo 1962
Lectures held at Advanced Study Institute of Feldspar - Oslo 1962

The only felspar for which we know accurately the orientation of the ellipsoïd of thermal expansion and the values of the principal expansion coefficients, is orthoclase.
In plagioclases a large rotation of the ellipsoïd occurs when the temperature rises.
The volume expansion, large for orthoclase and albite, is smaller for calcic felspars.
Research still in progress shows, that many orthoclases sharply expand at 580°-600 °0. This is ascribed to the release of stresses generally present in perthites. These stresses arise from the discrepancy between cell-dimensions in albite and orthoclase.
No such change occurs among peristerites.
A new method for the determination of the ellipsoid of thermal expansion in felspars is proposed. Use is made of the crackling varnishes (vernis craquelants), photoelastic varnishes, and strain ganges.
The method was first applied to an adularia of Switzerland.