The OH-F substitution in synthetic pargasite at 1.5 kbar, 850 degrees C

Amphiboles were synthesized at 1.5 kbar P-H2O and 850 degrees C along the j oin pargasite-fluoropargasite. Structural variations in the amphibole as a function of F have been characterized by a combination of SEM-EDS, X-ray po wder diffraction, and infrared spectroscopy. SEM-EDS analyses show that, fo r increasing F in the system, there is a decrease in Al-tot in the amphibol e and a significant decrease of F incorporation in the structure. In agreem ent with the EDS data, the variation in cell parameters and IR spectra show that incorporation off in pargasite is restricted to about 1.0 atoms per f ormula unit (apfu). The OH-stretching spectra show fine structure caused by F replacing OH at the O3 anion site, and are consistent with two-mode beha vior typical of A-site-filled amphiboles. The agreement between calculated and observed relative band intensities suggests complete short-range disord er of OH and F at the O3 anion site.