Huangite from Okumanza, Gunma Prefecture, Japan

摘要

Huangite was found from the type locality of minamiite in sulphuric altered andesite from Okumanza, Gunma Prefecture, Japan. EDS analyses indicate that "alunite" crystals are composed of very minute intergrowth of alunite, natroalunite, minamiite and huangite. The domain corresponding to each species are 5 to 100 μm in width. In the present huangite the ranges of A-site cations in general formula of alunite group minerals, AB_3(SO_4)_2(OH)_6, vary from Ca_0.46(K+Na)_0.10(Na K) to Ca_0.35(K+Na)_0.26(K>Na) on the basis of Al_3, that is, they correspond to approximately 90～70/100 mole of huangite end member, Ca_0.5Al_3(SO_4)_2 (OH)_6. Distinct compositional gaps are observed between 55 to 80/100 Ca mole on Na-Ca join and 5 to 60/100 Ca mole on K -Ca join in K- Na-Ca diagram of alunite group mineral.