Ternary Feldspars: Two-Feldspar Assemblages at P(H2O) = 5 kbar

Rhian H. Jones*
Department of Geology, University of Manchester, Manchester
*Present address: Institute of Meteoritics, Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131, USA.

Abstract: In a study of the liquidus surface in the silica-undersaturated part of the system Q-An-Ne-Ks at P(H2O) = 5 kbar, several two-feldspar plus liquid assemblages were obtained. The compositions of the feldspars were determined by electron microprobe analysis. The products of short runs (one or two days) consisted of fine intergrowths of two feldspars which may have crystallized during the heating period, or in the early stages of the run. Runs of four days were of sufficient length to coarsen these intergrowths such that individual euhedral crystals could be observed. However, analyses of the feldspar crystals from the longer runs showed a greater amount of ternary solid solution than expected. This may be attributed to the persistence of a small quantity of the second feldspar intergrown in the crystals. The slope of the tie-line joining one of the inferred pairs has been shown to be reproducible for gel and crystalline starting materials, hence the slopes of the tie-lines are believed to represent their equilibrium slopes.

Keywords: feldspars • two-feldspar assemblages • crystallization

Mineralogical Magazine; June 1989 v. 53; no. 371; p. 347-355; DOI: 10.1180/minmag.1989.053.371.10
© 1989, The Mineralogical Society
Mineralogical Society (www.minersoc.org)