INFLUENCE OF F-ANIONS ON THE HABIT AND GROWTH-MECHANISM OF POTASSIUM-RICH FELDSPARS

Orthoclase-rich alkalic feldspar crystals were grown from melts next t o Orthoclase-Albite-Quartz eutectic composition at 2 kb in the presenc e of a surplus of a supercritical aqueous phase by isobaric cooling at a rate of approximately 10 degrees C/day. Some runs were made with KF -bearing fluid phase, in two runs the melt was formed during intensive thermoconvection of the fluid phase. Tabular crystals were only forme d from F-bearing melts, crystals grown from F-free melt were always fa n-shaped. We assume this Fact is due to a more easy supply of Al to th e growing crystal in melts with F content.