Yukonite and wallkilldellite-(Fe) from the Kiura mine, Oita Prefecture, Japan

Yukonite and wallkilldellite-(Fe) were discovered at the Uriya deposit, Kiura mine, Ume, Saiki City, Oita, Japan, as first occurrences in Japan. Both minerals partly cover the surface of quartz and arsenopyrite masses and are closely associated, forming layered radiating aggregates. The layers are divided roughly into yellowish-brown and reddish-brown layers, where the former is yukonite and the latter is wallkilldellite-(Fe). The X-ray diffraction (XRD) pattern of Kiura yukonite has sharp peaks compared with that from the type locality, indicating that it is well crystalline. The empirical formula of the yukonite from electron probe microanalyzer (EPMA) analysis on the basis of As = 3 is Ca_2.31Fe³⁺ _4.20 (AsO₄)_3.00(OH)_8.22•12.07H₂O. The empirical formula of the wallkilldellite-(Fe) on the basis of O = 20 is Ca_3.51Fe²⁺ _5.13(AsO₄)_4.55(OH)_3.64•12.46H₂O. The wallkilldellite-(Fe) from the Kiura mine is very weak against heat, and the XRD pattern changes at 50 °C. On the other hand, the XRD pattern of the yukonite changes at 150 °C, a temperature higher than those of yukonite at other localities, indicating that it is well crystalline. We also found the signature of a structural relationship between the yukonite and wallkilldellite-(Fe) based on XRD and transmission electron microscope (TEM) observations.