Manganese in Monoclinic Members of the Epidote Group: Piemontite and Related Minerals

Abstract

According to Mayo (1932), who provided a brief historical review of the names used for piemontite, the first researcher who described this mineral has been Cronstedt in 1758 who named it “röd Magnesia.” In 1790 Chevalier Napione analyzed the sample described by Cronstedt and termed it “Manganèse rouge.” On the basis of his chemical data Haüy designated the substance as “Manganèse oxidé violet silicifère” in 1801. Later, in his Traité de Mineralogie, Haüy (1822) adopted the name proposed by Cordier (1803) who first recognized the mineral as an “Èpidote manganésifere.” The name piedmontite was proposed in 1853 by Kenngott the basis of the type locality and more recently transformed into piemontite.
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\nAccording to the standard guidelines for mineral nomenclature, the name piemontite should be reserved to members of the ternary solid solution Ca2Al2(Mn,Fe,Al)(Si2O7)(SiO4)O(OH) that basically contains Mn3+ dominant at one site. Nonetheless, the use of this name for any monoclinic manganiferous epidote-group members showing the characteristic strong red-yellow-violet pleochroism is very common and probably convenient with special regard to petrographic purposes. Indeed, the color of manganian (i.e., Mn3+ bearing) epidote or clinozoisite ranges to red to pinkish, while manganoan (i.e., Mn2+ bearing) members do not exhibit the characteristic reddish hue