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 C 2/ m amphibole species. They can be related to previously described species via the homovalent exchanges M 1(Fe2+Mg−1) and M 2(Fe3+Al−1), and represent the compositions richest in Fe in this compositional space. The ideal formula of ferri-clinoferroholmquistite is A □ B Li2 C (Fe3+ 2Fe2+ 3) T Si8 O22 X (OH)2, and that of sodic-ferri-ferropedrizite is A Na B Li2 C (Fe3+ 2Fe2+ 2Li) T Si8 O22 X (OH)2, with Z = 2. Holotype ferri-clinoferroholmquistite (IMA–CNMMN 2001–066), A (Na0.28K0.00)∑0.28 B (Li1.90Na0.08Ca0.02)∑2.00 C (Fe3+ 1.68Fe2+ 1.55Mg1.06Li0.28Al0.26Ti0.01Mn2+ 0.14Zn0.02)∑5.00 T Si8.00 O22O3(OH1.79F0.24)∑2.03, has a 9.462(6), b 17.898(9), c 5.302(3) A, β 101.88(4)°, V 878.6(2) A3. Holotype sodic-ferri-ferropedrizite (IMA–CNMMN 2001– 068), A (Na0.53K0.03)∑0.56 B (Li1.82Na0.15Ca0.03)∑2.00 C (Fe3+ 1.55Fe2+ 1.49Mg0.95Li0.58Al0.21Ti0.13 Mn2+ 0.07Zn0.01)∑4.99 T Si8.00 O22 X (OH1.45F0.52)∑1.97, has a 9.496(4), b 17.883(8), c 5.297(2) A, β 102.06(3)°, V 879.6(2) A3. 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 B Li and C Li 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 ( M 4′Li–O3F– Am Na) > ( M 4′Li–O3OH– Am Na) of preferred local configurations.