Ferri-ottoliniite and ferriwhittakerite, two new end-members of the new Group 5 for monoclinic amphiboles

摘要

Mineralogical and crystal-chemical descriptions are provided for two end-members of the new Group 5 introduced by Leake et al. (2004) in the nomenclature of amphiboles. This new classification scheme recognizes the occurrence of compositions with B-site populations intermediate between (Ca,Na) amphiboles and (Mg,Fe,Mn,Li) amphiboles. Its implications for the classification and nomenclature of Li-rich amphiboles are also discussed in this paper. Holotype ferri-ottoliniite [ideally AB(NaLi) C(Mg₃Fe₂3+) Si₈ O₂₂ (OH)₂] has a = 9.535(3), b = 17.876(6), c = 5.294(2) Å, ß = 102.54°(1), V = 880.9 Å3, and unit formula A(K_0.07Na_0.38)B(Na_0.70Li_1.24Ca_0.06)C(Mg_1.35Fe_0.922+Mn_0.132+ Zn_0.31Fe_1.713+Al_0.10Ti_0.06Li_0.42) Si₈ O₂₂ (OH_1.51F_0.47). Holotype ferriwhittakerite [ideally ANa B(NaLi) C(Mg₂Fe₂3+Li) Si₈ O₂₂ (OH)₂] has a = 9.712(9), b = 17.851(23), c = 5.297(2) Å, ß = 103.63°(5), V = 892.5 Å3, and unit formula A(K_0.13Na_0.64) B(Na_1.27Li_0.62Ca_0.11) C(Mg_1.47Fe_0.582+ Mn_0.122+ Zn_0.40Fe_1.483+ Al_0.10Ti_0.12Li_0.73) Si₈ O₂₂ (OH_1.30F_0.72). The root names have been chosen to acknowledge the contribution given to crystal-chemical studies of minerals, and particularly of amphiboles, by L. Ottolini (Pavia, Italy) and E.J.W. Whittaker (Oxford, U.K.).