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 “rod Magnesia.” In 1790 Chevalier Napione analyzed the sample described by Cronstedt and termed it “Manganese rouge.” On the basis of his chemical data Hauy designated the substance as “Manganese oxide violet silicifere” in 1801. Later, in his Traite de Mineralogie, Hauy (1822) adopted the name proposed by Cordier (1803) who first recognized the mineral as an “Epidote manganesifere.” The name piedmontite was proposed in 1853 by Kenngott the basis of the type locality and more recently transformed into piemontite. According 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. The discredited name “withamite” was used to describe poorly manganiferous piemontite (Hutton 1938; Yoshimura and Momoi 1964) but corresponds, on the basis of the current nomenclature, to a manganian clinozoisite. The name “thulite,” sometimes erroneously used for pinkish clinozoisite, should be reserved to Mn3+ bearing orthorhombic members. In this chapter we focus on piemontite sensu stricto with the ideal formula Ca2Al2Mn3+(Si2O7)(SiO4)O(OH), but also include for the reasons explained above those members of the clinozoisite-epidote-piemontite …