Oberti R , Hawthorne F C , Camara F , Raudsepp M , American Mineralogist , 84 (1999) p.102-111, Unusual M3+ cations in synthetic amphiboles with nominal fluoro-eckermannite, composition: Deviations from stoichiometry and structural effects of the, cummingtonite component, Sample: Al, Na.98Na1.10Mg.90Mg4.88Al.12Si8O22F2