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* Dipartimento di Scienze della Terra, Università degli Studi della Calabria, 1–87036 Arcavacata di Rende (CS), Italy
** ISTO (CNRS-Université d'Orléans), 1A rue de la Férollerie, F-45071 Orléans Cedex 2, France
*** Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
e-mail: dellaven{at}unical.it
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)O22(OH,F)2]. Syntheses have been done at 3 kbar PH20 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 M1MgM1MgM3Mg configurations are preferentially replaced by F whereas those OH-groups locally bonded to M1MgM1MgM3Al configurations are not replaced by F.
Key-words: richterite, pargasite, EMPA, cell parameters, infrared spectroscopy, OH-F incorporation.
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