Asagiite, NiCu₄(SO₄)₂(OH)₆·6H₂O, a new member of the ktenasite group from the Nakauri mine, Shinshiro City, Aichi Prefecture, Japan

Abstract

Asagiite, a newly-discovered mineral having the ideal formula NiCu4(SO4)2(OH)6·6H2O, is a member of the ktenasite group, representing a Ni analogue. It occurs as a secondary mineral on smithsonite aggregates that overlie fractures in a serpentinite found in the Nakauri mine within Aichi Prefecture, Japan. Asagiite exhibits a unique pale blue-green coloration and so is named after the traditional Japanese color “asagi-iro.” Asagiite occurs as thin plate-like crystals with perfect cleavage along {001} planes. The crystal size of this mineral is typically 0.1 to 0.2 mm, although in rare cases crystals may range up to 0.5 mm in length. These crystals are vitreous, transparent and non-fluorescent and have also been shown to be brittle with a Mohs hardness of 2½. The measured and calculated densities of asagiite are 2.90(3) and 2.92 g·cm-3, respectively. This mineral is optically biaxial (-) with α = 1.577(2), β = 1.620(2) and γ = 1.631(2) together with a 2Vcalc value of 52.4°. Electron microprobe analyses determined an empirical formula (based on 2S) of (Cu3.44Ni0.76Zn0.59Co0.18Fe0.01)Σ4.98S2O7.95(OH)6.05·6H2O. Based on single crystal X-ray diffraction data, the structure is monoclinic with space group P21/c and unit cell parameters a = 5.6095(8), b = 6.1259(7), c = 23.758(3) Å, β = 95.288(4)°, V = 812.92(17) Å3 and Z = 2. Single-crystal structural determination also gives an R1 value of 0.0303. The seven most intense peaks in the powder X-ray diffraction pattern [d in Å (I/I0) hkl] were found to be 11.830 (100) 002, 5.912 (64) 004, 4.845 (55) 013, 3.920 (45) 006, 2.953 (33) 008, 2.668 (57) 202 and 2.571 (36) 123, with unit cell parameters of a = 5.614(5), b = 6.108(8), c = 23.758(18) Å, β = 95.62(7)° and V = 810.8(14) Å3.