Crystal chemical investigations of epidote group minerals from two Bulgarian localities: Effects of Mn content and Mn/Fe ratio on the structural peculiarities

The current study presents results of crystal chemical studies of epidote samples from two Central Rhodope localities. Based on the obtained crystal chemical information, the nomenclature of the investigated species has been justified within the IMA-recommended nomenclature of epidote-group minerals. A comparative study has also been accomplished, including data from the present study as well as literature data from selected epidotes possessing similar crystal structural peculiarities. Certain trends have been derived from the transition metal(s) type and content(s) in the most distorted of the three six-coordinated sites (designated as M3) on selected geometrical and crystallographic parameters. The impact of Mn content and Mn/Fe ratio on the structural peculiarities and coloration of the specimens containing this element are discussed in terms of manifestation of the Jahn-Teller effect on the herein-considered epidote phases.