Karlditmarite (IMA 2021-003), Cu9O4(PO4)2(SO4)2, is a new mineral species from an active Arsenatnaya fumarole, Tolbachik volcano, Kamchatka peninsula. Karlditmarite occurs as green prismatic crystals. The mineral is biaxial (−), with α = 1.872(2), β = 1.835(3), and γ = 1.810(3) (589 nm). Under the microscope, karlditmarite is green with weak pleochroism. Electron microprobe analysis provided the empirical formula (Cu8.614Zn0.175Al0.053Ca0.019Fe0.157)(P1.574S1.814As0.444V0.109Si0.059)O20. Karlditmarite is triclinic, P−1: a = 6.1256(7) Å, b = 7.9192(8) Å, c = 7.9866(8) Å, α = 75.173(2)°, β = 86.639(2)°, γ = 88.660(2)°, V = 373.87(7). The crystal structure (R1 = 0.039) is unique. The [Cu9O4]10+∞2 layer in karlditmarite can be described as composed of six-membered rings, in which two of the six OCu4 tetrahedra share a common edge. The interlayer space between the bends of the highly corrugated [Cu9O4]10+∞2 layers hosts phosphate tetrahedra, whereas sulfate tetrahedra are placed above the centers of the rings. Karlditmarite is the first anhydrous Cu phosphate-sulfate mineral among more than one hundred copper oxysalt mineral species known from the active fumaroles.Phosphorus geochemistry in fumarolic environments is discussed.
