NOMENCLATURE OF AMPHIBOLES: ADDITIONS AND REVISIONS TO THE INTERNATIONAL MINERALOGICAL ASSOCIATION'S 1997 RECOMMENDATIONS

Abstract

The introduction of a fifth group of amphiboles, the Na–Ca–Mg–Fe–Mn–Li group, defined by 0.50 < B(Mg,Fe2+,Mn2+,Li) < 1.50 and 0.50 ≤ B(Ca,Na) ≤ 1.50 atoms per formula unit, with members whittakerite and ottoliniite, has been required by recent discoveries of B(Li,Na) amphiboles. These, and other new discoveries, such as sodicpedrizite (which is herein slightly, but significantly, changed from the original idealized formula), necessitate amendments to the IMA 1997 definitions of the Mg–Fe–Mn–Li, calcic, sodic-calcic and sodic groups. The discovery of obertiite and the finding of an incompatibility in the IMA 1997 subdivision of the sodic group require further amendments within the sodic group. All these changes, which have IMA approval, are summarized.