Kernowite, Cu2Fe(AsO4)(OH)4⋅4H2O, the Fe3+-analogue of liroconite from Cornwall, UK

The occurrence, chemical composition and structural characterisation of the new mineral kernowite, ideally Cu2Fe(AsO4)(OH)4⋅4H2O, the Fe3+-analogue of liroconite, Cu2Al(AsO4)(OH)4⋅4H2O, are described. Kernowite (IMA2020-053) occurs on specimens probably sourced from the Wheal Gorland mine, St Day, Cornwall, UK, in the cavities of a quartz-gossan rich in undifferentiated micro-crystalline grey sulfides and poorly crystalline arsenic phases including both pharmacosiderite and olivenite-group minerals. The average composition of kernowite determined from several holotype fragments by electron microprobe analysis is Cu1.88(Fe0.79Al0.09)Σ0.88(As1.12O4)(OH)4⋅3.65H2O. The structure of kernowite has been determined in monoclinic space group I2/a (a non-standard setting of C2/c) by single-crystal X-ray diffraction (SCXRD) to R1 = 0.025, wR2 = 0.051 and Goodness-of-fit = 1.112. Unit-cell parameters from SCXRD are a = 12.9243(4) A, b = 7.5401(3) A, c = 10.0271(3) A, β = 91.267(3)°, V = 976.91(6) A3 and Z = 4. The chemical formula of this crystal indicated by SCXRD from refined site-scattering is Cu2(Fe3+0.84(1)Al0.16)AsO4(OH)4⋅4H2O. The network of hydrogen bonding has been determined and is similar to that reported for liroconite from Wheal Gorland by Plumhoff et al. (2020).