Original paper

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

Banno, Yasuyuki; Miyawaki, Ritsuro; Matsubara, Satoshi; Makino, Kuniaki; Bunno, Michiaki; Yamada, Shigeo; Kamiya, Toshiaki

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 associated with phlogopite, titanite, calcite, pyrrhotite and chalcopyrite, and 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. The crystal structure of magnesiosadanagaite was refined for two compositionally different samples. Magnesiosadanagaite has the lowest O(5)-O(6)-O(5) angles (160.4° and 161.8°) ever reported in C2/m amphiboles, indicating maximal kinking of the double-chain of tetrahedra.

Keywords

key-words magnesiosadanagaitenew mineral speciessi-poor amphibolestructure refinementjapan