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(Na_0.6Ca_1.4)(Mg_4.6Al_0.4)(Si_7.2Al_0.8)O₂₂(OH,F)₂]. Syntheses have been done at 3 kbar P_H20 and 900°C, 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 ^[6]Al content decreases to 0.13 apfu for the nominal fluorine end-member whereas the ^[4]Al 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 ^M1Mg^M1Mg^M3Mg configurations are preferentially replaced by F whereas those OH-groups locally bonded to ^M1Mg^M1Mg^M3Al configurations are not replaced by F.