FERRI-OTTOLINIITE AND FERRIWHITTAKERITE, TWO NEW END-MEMBERS OF THE NEW GROUP 5 FOR MONOCLINIC AMPHIBOLES

Abstract

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 A□B(NaLi) C(Mg3Fe23+) Si8 O22 (OH)2] has a = 9.535(3), b = 17.876(6), c = 5.294(2) Å, β = 102.54°(1), V = 880.9 Å3, and unit formula A(K0.07Na0.38)B(Na0.70Li1.24Ca0.06)C(Mg1.35Fe0.922+Mn0.132+ Zn0.31Fe1.713+Al0.10Ti0.06Li0.42) Si8 O22 (OH1.51F0.47). 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, β = 103.63°(5), V = 892.5 Å3, and unit formula A(K0.13Na0.64) B(Na1.27Li0.62Ca0.11) C(Mg1.47Fe0.582+ Mn0.122+ Zn0.40Fe1.483+ Al0.10Ti0.12Li0.73) Si8 O22 (OH1.30F0.72). 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.).