Nb-Ta minerals in a lithium pegmatite from Myokenzan, Ibaraki Prefecture, Japan

Abstract

Visibly zoned grains composed of amber yellow rim and grey black core in a lepidolite-bearing pegmatite from Myokenzan, Ibaraki Prefecture, consist of microlite plus uranmicrolite rim and of stibiotantalite core. The second and third are new to Japan. The associated minerals include lepidolite, albite, quartz and the members of manganocolumbite-manganotantalite series. Microlite is high in Na2O content, and compositionally close to the ideal formula NaCa(Ta,Nb)₂O₆(F, OH). Uranmicrolite is also high in Na₂O and UO₂ contents yielding the relation Na>U>Ca. This relation is unique to this species. Stibiotantalite is niobian with Ta : Nb = 3 : 1. The chemical compositions of manganocolumbite-manganotantalite series are around the point of Nb : Ta = 1 : 1 with minor fluctuation, requiring two names. The unit cell parameters of microlite and stibiotantalite are; a=10.440Å and a=4.930, b=11.821, c=5.548Å respectively. Uranmicrolite rims microlite and uranium-rich microlite fills cracks therein. This is interpreted as the later incorporation of uranium-bearing material and its fixation after the formation of microlite.