Abstract
Talmessite was found in veinlets (approximately 1 mm wide) cutting into massive limonite in the oxidized zone of the Uriya deposit, Kiura mining area, Oita Prefecture, Japan. It occurs as aggregates of granular crystals up to 10 μm in diameter and as botryoidal aggregates up to 0.5 mm in diameter, in association with arseniosiderite, and aragonite. The talmessite is white to colorless, transparent, and has a vitreous luster. The unit-cell parameters refined from powder X-ray diffraction patterns are a = 5.905(3), b = 6.989(3), c = 5.567(4) Å, α = 96.99(3), β = 108.97(4), γ = 108.15(4)°, and Z = 1. Electron microprobe analyses gave the empirical formula Ca2.15(Mg0.84Mn0.05Zn0.02Fe0.01Co0.01Ni0.01)∑0.94(AsO4)1.91·2H2O on the basis of total cations = 5 apfu (water content calculated as 2 H2O pfu). It is suggested that the talmessite formed as a secondary mineral derived from löllingite, calcite, and diopside.
