SYNTHESIS OF FLUORPHLOGOPITE SINGLE-CRYSTALS - APPLICATIONS TO EXPERIMENTAL STUDIES

We describe a practical method of synthesis of fluorphlogopite single crystals suited to common laboratory conditions. The l-atm batch melti ng technique was adapted using starting compositions having a slight e xcess of fluorine, added as K2SiF6 The oxide-fluoride mixture was melt ed above the liquidus temperature and then cooled at a rate of a few d egrees per hour between 1400 and 1300 degrees C. Large (millimetre- to centimetre-size), clear and detachable single crystals were obtained by adding a quench of the charge during the cooling stage. XRD, TEM an d electron microprobe analyses confirm that the micas produced are mos tly monoclinic 1M polytypes having chemical compositions very close to the end-member fluorphlogopite. Heating experiments confirm that melt ing of fluorphlogopite is congruent. From both heating experiments and thermal analysis, the melting temperature of fluorphlogopite is locat ed at values greater than or equal to 1390 degrees C. Synthetic fluor- mica single crystals are of practical interest as analytical standards , and for several applications in experimental petrology.