Amphiboles cation ordering

Ernst W. G. and Wai C. M. (1970). Infrared, X-ray and optical stndy of cation ordering and dehydrogenation in natural and heat-treated sodic amphiboles. Amer. Mineral, 55 1226-1258. 

In alkali amphiboles, in which sodium fills the M4 positions, trivalent cations (Fe3+, Al3+) are largely confined to the smaller distorted octahedral M2 sites, so that the ordering pattern for Fe2+ ions becomes M3 > Ml M2. In the manganese-bearing alkali amphiboles winchite and juddite, Mn3+ ions are ordered in the M2 sites (Ghose et al., 1986), in which they are stabilized by the Jahn-Teller effect ( 6.3). 

---