Single crystals of ␥-Cu 2 V 2 O 7 were obtained by the method of chemical transport reactions. The crystal structure [triclinic, P1, a 5.0873(10), b 5.8233(11), c 9.4020(18) Å, ␣ 99.780(3), ␤ 97.253(3), ␥ 97.202(3)°, V 269.20(9) Å 3 , Z = 2] has been solved by direct methods and refined to R 1 = 0.021 (wR 2 = 0.049) using 949 unique observed reflections with |F o | ≥ 4 F . The atomic arrangement contains two symmetrically independent Cu 2+ cations. The Cu(1) site is coordinated by six O atoms to form a distorted Cu(1)O 6 octahedron, whereas the Cu (2) site is coordinated by five O atoms arranged at the vertices of an elongate Cu(2)O 5 square pyramid. Two symmetrically independent V 5+ cations are tetrahedrally coordinated by four O atoms each. The O(5) atom bridges the V(1)O 4 and V(2)O 4 tetrahedra. The Cu(1)O 6 octahedra and Cu(2)O 5 square pyramids form two types of chains running parallel to the a axis. The chains are linked by V 2 O 7 groups into a complex heteropolyhedral framework. The structure of ␥-Cu 2 V 2 O 7 is closely related to the structures of ␣-Cu 2 V 2 O 7 (blossite) and ␤-Cu 2 V 2 O 7 (ziesite). The three structures are based upon chains of edge-sharing Cu 2+ polyhedra linked by V 2 O 7 groups. Similarity of the structure of ␥-Cu 2 V 2 O 7 to the structures of blossite and ziesite indicates possible formation of this phase under natural conditions, such as those that occur in "dry" high-temperature fumaroles of the Izalco (El Salvador) and Tolbachik (Kamchatka, Russia) volcanoes. The structure of ␥-Cu 2 V 2 O 7 can also be considered as a triclinically distorted derivative of the ␤-A 2 P 2 O 7 structure-type.

The crystal structure of the unstable mineral alumoklyuchevskite K 3 Cu 3 AlO 2 (SO 4 ) 4 [monoclinic, I 2, a = 18.772(7), b = 4.967(2), c = 18.468(7) Å, β = 101.66(1) ° , V = 1686(1) Å] was refined to R 1 = 0.131 for 2450 unique reflections with F ≥ 4 σ F hkl . The structure is based on oxocentered tetrahedrons (OAl ) linked into chains via edges. Each chain is surrounded by SO 4 tetrahedrons forming a structural complex. Each complex is elongated along the b axis. This type of crystal structure was also found in other fumarole minerals of the Great