The behaviour of Mn in amphiboles: Mn in synthetic fluor-edenite and synthetic fluor-pargasite

The crystal structures of two synthetic C2/ra amphiboles, manganoan fluor-edenite Nai.o6(Cai.4iNao.i2Mno.47)(Mg4.44Mno.4ilAlo.i5)(Si6.9iAli.o9)O22F2, a = 9.829(4), b = 18.024(9), c = 5.284(2) Å, ß = 104.66(2)°, and manganoan fluor-pargasite (Nao.96Cao.13XCa1.51Mno.49) (Mg3.47Mno.73Alo.8o)(Sis.95Al2.o5)022F2, a = 9.812(4), b = 17.926(6), c = 5.308(3) Å, ß = 105.10(3)°, have been refined to R indices of 2.0 % and 1.4 %, respectively, using MoKa X-ray data. Site-scattering refinement and electron-microprobe data both indicate that there is a large amount (~ 0.50 apfu) of Mn2+ at the M(4) site in each synthetic amphibole. C-group Mn2+ shows the ordering pattern M(2) » M(l) » M(3) in these amphiboles as in many natural sodic-calcic and alkali amphiboles. This behaviour is significantly different from that of Fe2+ which shows the ordering pattern M(l) < M(3) in Mn-poor calcic, sodic-calcic and alkali amphiboles. The presence of significant Ca at the A site in F-rich pargasite is also confirmed.