A new hyper-calcic amphibole with Ca at the a site: Fluor-cannilloite from Pargas, Finland

Electron microprobe analysis of amphiboles from Pargas, Finland, shows up to 2.42 Ca apfu (atoms per formula unit), far in excess of the usual maximum value of 2.00 Ca apfu observed in amphiboles. The most Ca-rich composition is (Ko.12Nao.35Cao.52)(Cal.92- Mn6.65 Fe6.63)(Mg3.73 Fe6.t3 Tio.o2Alo.82)(Si5.63AI2.37 )022 [(OH)0.46F 1.54]. Fluor-cannilloite, ideally CaCa2(Mg4AI)(Si5AI3)022F2, is a new amphibole species in which Ca is dominant at the A site. Fluor-cannilloite occurs as isolated anhedral grains and small granular aggregates in a marble together with calcite, muscovite, anorthite, aluminous diopside, pyrope, and fluorite. It is grayish green with a grayish white streak, brittle, H = 6, Dmeas= 3.05 g/cm3, and shows perfect {110} cleavage. In plane-polarized light, it is pale greenish gray to almost colorless, rather cloudy and mottled in appearance, and is not pleochroic. Fluor-cannilloite is biaxial positive, a = 1.611(2), (3= 1.616(2), l" = 1.633(2), 2 V = 49(2)°, dispersion weak r > v. It is monoclinic, space group C21m, a = 9.826(4), b = 17.906(9), c = 5.301(3) A, (3 = 105.41(4)°, V = 899.2 A3, Z = 2. The ten strongest X-ray diffraction lines in the powder pattern [d in A(I)(hkl)] are 8.936(4)(020), 8.355(3)(110), 3.366(4)(131), 3.107(7)(310),2.686(10)(151), 2.578(10)(061), 2.328(4)(351), 2.165(6)(171), 2.036(5)(202), 1.435(9)(4.10.1 ). The structures of fluor-cannilloite and another Ca-rich amphibole from Madagascar were refined to R indices of -.-1.5°10using reflection intensities collected with MoKa X radiation. Site populations were assigned from the refined site-scattering values, considerations of mean bond lengths, and the unit formulas calculated from the electron microprobe analyses. Both the electron microprobe data and the site-scattering refinements show significant Ca (up to 0.52 apfu) together with Na and K at the A site. Both A2 and Am sites are occupied, with the A2 site having the higher electron density. Local bond-valence considerations indicate that Ca is equally partitioned between the A2 and Am sites, whereas K occupies Am and Na occupies A2.