New amphibole compositions: natural and synthetic

The crystal-chemical formula and the major-element composition of amphiboles have been discussed in Hawthorne and Oberti (2007a), and the amphibole compositional space has been defined in terms of root-names and allowed charge arrangements in Hawthorne and Oberti (2007b). Although the number of possible homovalent substitutions occurring at the various structural sites is very high, geochemically exotic cations generally do not reach the limits required to assign a distinct prefix to the root-name. Thus prefixes are so far confined to the presence of Na, K, Mg, Al, Ti, Mn, Fe, F and Cl. In this chapter, we examine the new amphibole compositions which have been found and characterized since 1981, i.e. since the publication of volume 9A of the MSA Reviews in Mineralogy. We divide the chapter in two sections. The first section describes new charge arrangements found in Nature, and the second section describes the extensive studies of synthetic amphiboles of novel composition done to clarify the crystal chemistry of exotic cations which are normally found as minor components. This chapter deals exclusively with monoclinic amphiboles, as the compositional space of orthorhombic amphiboles has not expanded since the former review. Although natural occurrences of protoamphibole (with space group Pnmn) have been described only in the last twenty years (Sueno et al. 1998; Konishi et al. 2003), their composition is within the compositional range previously defined for Pnma amphiboles.