Sodic-ferri-ferropedrizite and ferri-clinoferroholmquistite : mineral data and degree of order of the A-site cations in Li-rich amphiboles

A systematic examination of metamorphic episyenites in the Pedriza Massif, Sierra de Guadarrama, Spain, allowed detection of two more distinct Li-rich C2/m amphibole species. They can be related to previously described species via the homovalent exchanges (M1)(Fe2+Mg-1) and (M2)(Fe3+Al-1, and represent the compositions richest in Fe in this compositional space. The ideal formula of ferri-clinoferroholmquistite is (A)boxed dot(B)Li(2) (C)(Fe23+Fe32+) Si-T(8) O-22 (X)(OH)(2), and that of sodic-ferri-ferropedrizite is Na-A Li-B(2) (C)(Fe23+Fe22+Li)Si-T(8) O-22 (X)(OH)(2), with Z = 2. Holotype ferri-clinoferroholmquistite (IMA-CNMMN 2001-066), (A)(Na0.28K0.00)(Sigma0.28) B(Li1.9Na0.08Ca0.02)(Sigma2.00) (C)(Fe1.683+Fe1.552+Mg1.06Li0.28Al0.26Ti001Mn0.142+Zn0.02)(Sigma5.00) Si-T(8.00) O-22 (O3)(OH1.79F0.24)(Sigma2.03), has a 9.462(6), b 17.898(9), c 5.302(3) Angstrom, beta 101.88(4)degrees, V 878.6(2) Angstrom(3). Holotype sodic-ferri-ferropedrizite (IMA-CNMMN 2001068), (A)(Na0.53K0.03)(Sigma0.56) (B)(Li1.82Na0.15Ca0.03)(Sigma2.00) (C)(Fe1.553+Fe1.492+Mg0.95Li0.58Al0.21Ti0.13Mn0.072+Zn0.01)(Sigma4.99) Si-T(8.00) O-22 (X)(OH1.45F0.52)(Sigma1.97), has a 9.496(4), b 17.883(8), c 5.297(2) Angstrom, beta 102.06(3)degrees, V 879.6(2) Angstrom(3). Sodic-ferri-ferropedrizite and ferriclinoferroholmquistite are both black, with a vitreous luster. They have a gray streak and do not show fluorescence under long- and short-wave ultraviolet light; they are brittle with an uneven fracture, and have the characteristic {110} cleavage of monoclinic amphiboles. The calculated values of density are 3.23 and 3.24 g/cm(3), and the calculated mean indices of refraction are 1.708 and 1.710, respectively. The shape of the electron density at the A sites changes as a function of the Li-B and Li-C contents, suggesting that cation ordering is strongly affected by local bond-valence requirements. This feature is confirmed by systematic bond-valence-bond-strength calculations, which support the sequence (Li-M4'-F-O3-Na-Am), (Li-M4'-(OH)-O-O3-Na-Am) of preferred local configurations.