Petersite–(La), a new mixite–group mineral from Ohgurusu, Kiwa, Kumano City, Mie Prefecture, Japan

Petersite–(La) is a new mineral of the petersite series in the mixite group with an ideal formula of Cu6La(PO4)3(OH)6·3H2O from Ohgurusu, Kiwa–cho, Kumano City, Mie Prefecture, Japan. The mixite–group minerals occur in small cavities coated by chrysocolla developed along quartz veins. Four members from different cavities have been identified: petersite–(La), petersite–(Ce), petersite–(Y), and agardite–(La). Petersite–(La) occurs as a radial aggregate formed by acicular to hexagonal columnar crystals of yellowish green color. Crystals are elongated along [001] and the prismatic face is probably formed by {001} and {100} or {110}. It is non–fluorescent in UV light. Crystals are brittle, cleavage and parting are non–observed, and fracture is uneven. These characteristics are common in other mixite–group minerals. The calculated density of petersite–(La) is 3.33 g/cm3, based on the empirical formula and powder XRD data. It is optically uniaxial positive with ω = 1.680(3) and ε = 1.767(3) (white light), and pleochroism varies from light green to yellowish green. Based on the WDS analysis, the empirical formula of petersite–(La) calculated on the basis of P + As + Si = 3 is (Cu5.692Fe0.010)Σ5.702[(La0.148Ce0.122Nd0.117Y0.086Sm0.022)Σ0.495Ca0.372]Σ0.866(P1.890As0.799Si0.311)Σ3O10.320(OH)7.680·3H2O. Petersite–(La) is hexagonal (P63/m) with a = 13.367(2) Å, c = 5.872(2) Å, and V = 908.7(4) Å3 (Z = 2). The eight strongest lines of petersite–(La) in the powder XRD pattern [d in Å(I/I0)(hkl )] are 11.578(100)(100), 4.377(28)(210 + 120), 3.509(18)(211 + 121), 3.211(10)(310 + 130), 2.898(14)(221, 400), 2.656(10)(320 + 230), 2.526(11)(410 + 140), and 2.438(25)(212 + 122). Petersite–(La) is the third defined member in the petersite series and corresponds to the La–dominant analogue of petersite–(Y) and petersite–(Ce).