Magnesiosadanagaite, a new member of the amphibole group from Kasuga-mura, Gifu Prefecture, central Japan

Abstract

Magnesiosadanagaite, ideally NaCa2[Mg3(Al,Fe3+)2]Si5Al3O22(OH)2, is a new member of the amphibole group occurring as rims of zoned amphibole in rock samples from the contact aureole in Kasuga-mura, central Japan. The amphibole is composed of an [4]Al-poorer core and [4]Al-richer rim. The core and rim are classified into pargasite [[4]Al = 1.84-2.19 atoms per formula unit (apfu); for O = 23] and pargasite-magnesiosadanagaite ([4]Al = 2.45-2.84 apfu), respectively. Both have relatively high [A]Na (0.67-0.96 apfu) and low K content (< 0.29 apfu). The high [A]Na (0.70-0.92 apfu) and low K (0.09-0.28 apfu) values of magnesiosadanagaite refute the view that high K substitution at the A site is an essential feature of Si-poor calcic amphiboles.