Petersite-(Ce), cu ²⁺ ₆CE(PO₄)₃(OH)₆-3H₂O, a new mixite group mineral from yavapai county, Arizona, USA

A new mineral, petersite-(Ce), ideally Cu2 6Ce(PO4)3(OH)63H2O (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 20320350 lm 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 Cu_6.05(Ce_0.18Y_0.16La_0.12Nd_0.09Gd_0.03Pr_0.02 Dy_0.01Sm_0.01Ca_0.42)R_1.04[(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) A , c 5.8591(9) A , and V 886.8(4) A₃, 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 cu ²⁺ ₆CE(PO₄)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.