Mikenewite, the natural analogue of synthetic α-Mn2+(S4+O3)⋅3H2O, a new sulfite mineral from the Ojuela mine, Mapimí, Mexico

Abstract

Abstract A new mineral species, mikenewite (IMA2022-102), ideally Mn2+(S4+O3)⋅3H2O, has been discovered from the San Judas Chimney, Ojuela mine, Mapimí, Durango, Mexico. It occurs as spheres of platy crystals. Associated minerals include goethite, cryptomelane, adamite and lotharmeyerite. Mikenewite is yellowish in transmitted light, transparent with a white streak and vitreous lustre. It is brittle and has a Mohs hardness of 2½–3. Cleavage is perfect on {101}. The measured and calculated densities are 2.48(5) and 2.467 g/cm3, respectively. Optically, mikenewite is biaxial (+), with α = 1.606(5), β = 1.614(5), γ = 1.627(1) (white light), 2V(meas.) = 69(3)° and 2V(calc.) = 77°. An electron microprobe analysis yielded an empirical formula (based on 6 O apfu) of (Mn0.86Zn0.12Fe0.04Ca0.02)Σ1.04(S0.98O3)⋅3H2O, which can be simplified to (Mn,Zn,Fe)(SO3)⋅3H2O. Mikenewite is the natural analogue of synthetic α-Mn2+(S4+O3)⋅3H2O, as well as the Mn-analogue of albertiniite, Fe2+(S4+O3)⋅3H2O. It is monoclinic, with space group P21/n and unit-cell parameters a = 6.6390(3), b = 8.8895(4), c = 8.7900(4) Å, β = 96.095(2)°, V = 515.83(4) Å3 and Z = 4. The crystal structure of mikenewite is characterised by each Mn atom coordinated octahedrally by six O atoms, three from different sulfite O atoms and three from H2O molecules. Each S4+O3 group is bonded to three Mn atoms, resulting in a sheet parallel to (101) with the sheet composition of Mn2+(S4+O3)⋅3H2O. Such sheets, stacked along [10$\bar{1}$], are joined together by hydrogen bonds, accounting for the perfect cleavage of the mineral. Mikenewite is dimorphous with orthorhombic Pnma gravegliaite, as albertiniite is with fleisstalite. Its discovery from the Ojuela mine, which is particularly rich in Zn, implies the possibility of finding Zn-bearing sulfites there as well.