Sodic-ferripedrizite, a new monoclinic amphibole bridging the magnesium-iron-manganese-lithium and the sodium-calcium groups

Sodic-ferripedrizite, ideally Na(LiNa)(Fe23+Mg2Li)Si8O22(OH)(2), is the sec ond new amphibole end-member found in episyenites formed after cordierite-b earing porphyritic granites in the East Pedriza Massif (Central System, Spa in). It is green, vitreous, translucent, non-fluorescent and brittle, and h as gray streak, H(Mohs) = 6, splintery fracture, perfect {110} cleavage, D- meas = 3.15, D-calc = 3.15 g/ cm(3). In plane-polarized light, sodic-ferrip edrizite is strongly pleochroic, X = green blue, Y = blue green, Z = yellow green (X> Y> Z); Y = b, X <^> c = 4 degrees, Z <^> c = -8 degrees. It is b iaxial positive: alpha = 1.694 (1), beta = 1.698(1), and gamma = 1.702(1); 2V(z) = 83(2)degrees and 2V(Zcalc) = 85(3)degrees, dispersion r > v. Sodic- ferripedrizite is monoclinic. space group C2/m, an = 9.536(1), b = 17.789(1 ), c = 5.277(1) Angstrom, beta = 102.53 degrees, V = 873.8(1) Angstrom(3). The five strongest Lines in the X-ray powder-diffraction pattern [d in Angs trom[I] (hkl) are 3.397[3](131), 3.056[10](310), 2.749[5](330), 2.699[6](15 1), and 1.639[4](461). Analysis by electron microprobe and flame photometry gave an average chemical formula of <^>(Na0.70K0.03) (B)(Li1.34Na Ca-0.58( 0.08)) (Mg1.75Fe1.653+Li0.88Fe0.322+Al0.21Ti0.11Mn0.072+Zn0.01)Si8.00O22(OH 1.35F0.65). Structure refinement of one crystal of pedrizite and of another crystal with higher clinoholmquistite component, both of which were also a nalyzed by ion microprobe for light and volatile elements, allowed us to as sess Li partitioning among the B- and C-group sites and the active crystal- chemical mechanisms; Li is ordered at the [6+2]-coordinated position in the M4 cavity and at the M3 site. Thus, sodic-ferripedrizite encompasses the d ifferent site-preferences and crystal-chemical mechanisms observed for Li i n amphiboles of the Mg-Fe-Mn-Li group (clinoholmquistite series) and the Na -Ca group (leakeite series). These data and other recent results on synthet ic amphiboles suggest that miscibility between the two groups of B-cations is far more extensive than previously expected.