SODIC-FERRI-FERROPEDRIZITE AND FERRI-CLINOFERROHOLMQUISTITE: MINERAL DATA AND DEGREE OF ORDER OF THE A-SITE CATIONS IN LI-RICH AMPHIBOLES

Abstract

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□ BLi2 C(Fe3+2Fe2+3) TSi8 O22 X(OH)2, and that of sodic-ferri-ferropedrizite is ANaBLi2C(Fe3+2Fe2+2Li) TSi8 O22 X(OH)2, with Z = 2. Holotype ferri-clinoferroholmquistite (IMA–CNMMN 2001–066), A(Na0.28K0.00)Σ0.28 B(Li1.90Na0.08Ca0.02)Σ2.00 C(Fe3+1.68Fe2+1.55Mg1.06Li0.28Al0.26Ti0.01Mn2+0.14Zn0.02)Σ5.00 TSi8.00 O22 O3(OH1.79F0.24)Σ2.03, has a 9.462(6), b 17.898(9), c 5.302(3) Å, β 101.88(4)°, V 878.6(2) Å3 . Holotype sodic-ferri-ferropedrizite (IMA–CNMMN 2001–068), A(Na0.53K0.03)Σ0.56 B(Li1.82Na0.15Ca0.03)Σ2.00 C(Fe3+1.55Fe2+1.49Mg0.95Li0.58Al0.21Ti0.13 Mn2+0.07Zn0.01)Σ4.99 TSi8.00 O22X(OH1.45F0.52)Σ1.97, has a 9.496(4), b 17.883(8), c 5.297(2) Å, β 102.06(3)°, V 879.6(2) Å3. Sodic-ferri-ferropedrizite and ferri-clinoferroholmquistite 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/cm3 , 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 BLi and CLi 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 (M4’Li–O3F–AmNa) > (M4’Li–O3OH–AmNa) of preferred localconfigurations.