ANKERITE: ITS COMPOSITION AMD FORMULAE, AND ITS STATUS IN THE NORTHERN PENNINE OREFIELD

摘要

The composition field of the dolomite-group mineral ankerite is defined inconsistently in the geological literature. Guidelines introduced by the International Mineralogical Association (IMA) use the dominant element at a particular structural site to define species boundaries. The dolomite-group minerals have a general formula CaM~2(CO_3)_2, wherethe M~(2+) cations are commonly Mg~(2+), Fe~(2+) and Mn~(2+) in various combinations. IMA guidelines define ankerite as the dolomite-group species with Fe~(2+) as the dominant cation in the M~(2+) position. This definition is at variance with many of those used in the older geological literature. In recent years, most mineralogical journals have adopted IMA-approved species boundaries as standard and the differing historical definitions have the potential to introduce errors.