Petersite-(Ce), Cu2+6Ce(PO4)3(OH)6·3H2O, A New Mixite Group Mineral From Yavapai County, Arizona, USA

Abstract

Abstract A new mineral, petersite-(Ce), ideally Cu 2+ 6 Ce(PO 4 ) 3 (OH) 6 ·3H 2 O (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/cm 3 . An electron microprobe analysis resulted in an empirical chemical formula of Cu 6.05 (Ce 0.18 Y 0.16 La 0.12 Nd 0.09 Gd 0.03 Pr 0.02 Dy 0.01 Sm 0.01 Ca 0.42 ) Σ1.04 [(PO 4 ) 2.54 (SiO 4 ) 0.14 (PO 3 OH) 0.32 (OH) 6 ]·3.65H 2 O. Petersite-(Ce) is hexagonal, with space group P 6 3 / m and unit-cell parameters a 13.2197(18) A, c 5.8591(9) A, and V 886.8(4) A 3 , 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+ 6 A ( T O 4 ) 3 (OH) 6 · 3H 2 O, 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 CuO 5 square-pyramids along c . These chains are connected in the a-b plane by edge-sharing CeO 9 polyhedra and corner-sharing PO 4 tetrahedra. Hydroxyl groups occupy each corner of the CuO 5 polyhedra not shared by a neighboring P or Ce atom. Each CeO 9 polyhedron is surrounded by three zeolitic channels. The walls of the channels, parallel to c , are six-membered, hexagonal rings composed of CuO 5 and PO 4 polyhedra in a ratio of 2:1, respectively, and contain H 2 O molecules. In our model of petersite-(Ce), we defined one distinct H 2 O site positioned to form a ring inside the channel, although there are many statistically possible locations.