Jahnsite-(MnMnZn), a new jahnsite-group mineral, and formal approval of the jahnsite group

Jahnsite-(MnMnZn) (IMA2017-113), $Mn22+Zn2Fe23+(PO4)4(OH)2∙8H2O$⁠, is a new member of the jahnsite group from the Herdade dos Pendões mine, Beja district, southwest Portugal. It formed in a highly altered gossan in association with Zn-bearing libethenite, quartz, rhodochrosite and santabarbaraite. It occurs in sub-parallel bundles of light golden brown prisms up to 0.3 mm long. Crystals are transparent with vitreous to silky lustre and white streak. The mineral is brittle with irregular, splintery fracture, good cleavage parallel to {001} and Mohs’ hardness of about 4. The measured density is 2.89(2) g cm⁻³. Optically, it is biaxial (+), with α = 1.655(2), β = 1.662(2), γ = 1.673(2) (white light); 2V = 78(1)°; pleochroism: X nearly colourless, Y and Z beige; X < Y ≈ Z. Electron microprobe analyses gave the empirical formula $(Mn0.712+Na0.15Ca0.10)Σ0.96Mn1.002+Zn1.00Mn0.662+Fe0.212+Mg0.10Fe0.033+Σ2.00(Fe1.993+Al0.01)Σ2.00(P1.01O4)4(OH)2∙8H2O$⁠. Jahnsite-(MnMnZn) is monoclinic, P2/a, a = 15.222(6), b = 7.187(6), c = 10.028(5) Å, β = 111.746(16)°, V = 1019.0(11) ų and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [d_obs/Å (I) (hkl)]: 9.25 (63) (001), 5.00 (40) (210,$211¯$⁠,111), 4.648 (33) (002), 3.509 (41) (⁠$402¯$⁠), 2.842 (100) (022), 1.9984 (37) (422,$424¯$⁠), 1.9506 (30) (024) and 1.5853 (33) (820,$824¯$⁠). The jahnsite group has now been officially established by the Commission on New Minerals, Nomenclature and Classification. The general formula for jahnsite-group minerals is XM1M2₂M3₂(H₂O)₈(OH)₂(PO₄)₄; the ionic radii for the cation sites generally increase as follows: M3 < M2 < M1 < X, and the M3 site determines whether the species belongs to the jahnsite subgroup (M3 = Fe³⁺) or the whiteite subgroup (M3 = Al). The subgroup name is generally used as the species root name to be followed by a suffix of the form -(XM1M2). Using this as a guide for the jahnsite described herein results in the following site assignments: X = Mn, M1 = Mn, M2 = Zn, and M3 = Fe; and the name jahnsite-(MnMnZn). Jahnsite-(MnMnZn).