A new mineral, petersite-(Ce), ideally Cu2+6Ce(PO4)3(OH)6·3H2O (IMA2014-002), has been found in the Cherry Creek District of Yavapai County, Arizona, USA. It is a secondary alteration mineral associated with malachite, chlorite, a biotite phase, quartz, albite, orthoclase, hematite, chalcopyrite, and an uncharacterized hisingerite-like mineral. Petersite-(Ce) occurs as sprays of yellowish-green, acicular crystals approximately 20 × 20 × 50 μm in size. It has a white streak with vitreous luster. The mineral is brittle and has a Mohs hardness of ∼3.5; no cleavage or parting was observed. The calculated density is 3.424 g/cm3. An electron microprobe analysis resulted in an empirical chemical formula of Cu6.05(Ce0.18Y0.16La0.12Nd0.09Gd0.03Pr0.02Dy0.01Sm0.01Ca0.42)Σ1.04[(PO4)2.54(SiO4)0.14(PO3OH)0.32(OH)6]·3.65H2O.

Petersite-(Ce) is hexagonal, with space group P63/m and unit-cell parameters a 13.2197(18) Å, c 5.8591(9) Å, and V 886.8(4) Å3, Z = 2. It is the Ce analogue of petersite-(Y) and exhibits the mixite structure type. The mixite group can be expressed by the general formula Cu2+6A(TO4)3(OH)6·3H2O, where nine-coordinated A is a rare earth element, Al, Ca, Pb, or Bi, and T is P or As. The structure of petersite-(Ce) is characterized by chains of edge-sharing CuO5 square-pyramids along c. These chains are connected in the a-b plane by edge-sharing CeO9 polyhedra and corner-sharing PO4 tetrahedra. Hydroxyl groups occupy each corner of the CuO5 polyhedra not shared by a neighboring P or Ce atom. Each CeO9 polyhedron is surrounded by three zeolitic channels. The walls of the channels, parallel to c, are six-membered, hexagonal rings composed of CuO5 and PO4 polyhedra in a ratio of 2:1, respectively, and contain H2O molecules. In our model of petersite-(Ce), we defined one distinct H2O site positioned to form a ring inside the channel, although there are many statistically possible locations.

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