Feitknechtite and its Origin from Noda-Tamagawa Mine, Iwate Prefecture, Japan.

Abstract

Pyrochroite occurs abundantly as a main ore-forming mineral at the bedded manganese ore bodies of Noda-Tamagawa Mine. The upper part of the deposits has been oxidized by supergene solutions and consists of manganese dioxide minerals and mixture of feitknechtite and hausmannite (so-called hydrohausmannite). On the other hand, feitknechtite occurs rarely in fissures and small faults developed in the bedded ore bodies consists mainly of pyrochorite with a small amounts of hausmannite, rhodochrosite and manganosite. Veinlets of this mineral at -180m level of Kirihata ore body is not at all affected by supergene solutions. It occurs in a aggregates of brownish black platy crystals up to 3mm in diameter and is intimately associated with hausmannite, and observation under the microscope reveals not seldom the fact that this mineral is pseudomorph after pyrochroite. Accordingly, feitknechtite is considered to be hydrothermal alteration product of pyrochroite formed from the hydrothermal solution accompanied by the granitic intrusion. This oxidation from pyrochroite to mixture of feitknechtite and hausamannite will be attributed to the rising of Eh and the decreasing of pH of the hydrothermal solution at later stage of the hydrothermal mineralization.