Constraints on F vs. OH incorporation in synthetic [6]Al-bearing monoclinic amphiboles

In this work we have systematically replaced OH with F in an amphibole composition intermediate between richterite and pargasite [60% richterite and 40% pargasite in molar proportion; nominal composition Na(Na0.6Ca1.4)(Mg4.6Al0.4)(Si7.2Al0.8)O-22(OH,F)(2)]. Syntheses have been done at 3 kbar P-H2O and 900 degreesC, and for all compositions an assemblage of amphibole plus minor Na-phlogopite has been obtained. Crystal-chemical variations in the amphibole as a function of F have been characterized by a combination of EMPA, X-ray powder diffraction, and infrared spectroscopy. EMPA analyses show that for increasing F in the system (i) the Al-[6] content decreases to 0.13 apfu for the nominal fluorine end-member whereas the Al-[4] content is virtually constant and close to the nominal value; (ii) the fluorine content is systematically lower than expected. The infrared OH-stretching spectra show that those OH-groups locally bonded to (MgMgMg)-Mg-M1-Mg-M1-Mg-M3 configurations are preferentially replaced by F whereas those OH-groups locally bonded to (MgMgAl)-Mg-M1-Mg-M1-Al-M3 configurations are not replaced by F.