Pseudolyonsite, ideally Cu3(VO4)2, is a new mineral from the medium-temperature fumaroles of the New Tolbachik scoria cones, Tolbachik volcano, Kamchatka Peninsula, Russia. It occurs as needles that are 5–20 μm across and up to 0.5 mm in the length, which sometimes produce parallel intergrowths, sprays or openwork clusters up to 2 mm. Associated minerals are: piypite, hematite, magnetite, lyonsite, aphthitalite, palmierite, langbeinite, filatovite, lammerite, vergasovaite, rutile and native gold. Pseudolyonsite is dark red with a brownish tint to black, translucent to opaque, with a reddish-brown streak and adamantine to semi-metallic lustre. The mineral is brittle, but thin long needles are flexible. The fracture is conchoidal, and no cleavage was observed. The calculated density is 4.749 g/cm3. In reflected light in air the mineral is grey with a weak bluish tint, non-pleochroic, has distinct anisotropy and ubiquitous red to orange internal reflections. The reflectance values (R1 and R2, %) in air for the four COM wavelengths are, respectively, 17.05, 19.6 (470 nm); 16.1, 18.15 (546 nm); 15.85, 17.7 (589 nm); and 15.55, 17.4 (650 nm). Four electron probe (EDS) analyses produced the following mean values: V2O5 40.37, CuO 48.83, ZnO 7.60, MoO3 1.89, and SiO2 0.14, total 98.83 wt%, which corresponds, on the basis of 8 O atoms, to (Cu2.58Zn0.44)∑3.02(V1.88Mo0.06Si0.02)∑1.96O8. The idealised formula is Cu3(VO4)2. Pseudolyonsite is monoclinic: P21/c, a = 6.2695(4), b = 8.0195(3), c = 6.3620(3) Å, β = 111.96(1)°, V = 296.66(3) Å3, Z = 2. The strongest powder X-ray diffraction lines [d in Å (I) (hkl)] are: 4.70 (60) (110); 3.30 (79) (021, 120); 3.22 (87) (111); 3.18 (34) (−121, −102); 2.894 (74) (200, −211); 2.761 (100) (012); 2.479 (59) (−212, −122); 2.419 (67) (031, 130). The crystal structure was solved from single-crystal data and refined to R = 0.0444. Pseudolyonsite is isostructural with synthetic monoclinic Cu3(VO4)2. The crystal structure of pseudolyonsite contains corrugated octahedral layers formed by the chains of edge-shared, distorted Cu(2)-octahedra running along the c axis and connected to each other by distorted Cu(1)-octahedra. The octahedra of both types contain Cu and subordinate Zn, and they are typically Jahn-Teller-distorted. Adjacent octahedral layers are connected to each other by VO4 tetrahedra. Pseudolyonsite is dimorphous with triclinic mcbirneyite. The name pseudolyonsite comes from its close visual similarity to another vanadate mineral, lyonsite, Cu3Fe3+4(VO4)6. Both the mineral and its name have been approved by the IMA CNMNC (IMA No. 2009-062).

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