Pieczkaite, ideally Mn5(PO4)3Cl, a new apatite-supergroup mineral from Cross Lake, Manitoba, Canada: Description and crystal structure

Pieczkaite, ideally Mn5(PO4)3Cl, is a new apatite-supergroup mineral from Cross Lake, Manitoba, Canada. It occurs as small patches and narrow veins in large crystals of apatite and (Mn,Cl)-bearing apatite in phosphate pods in the quartz core of a granitic pegmatite. Veins of Mn-bearing apatite narrow to ~25 μm where the Mn content becomes high enough to constitute pieczkaite. It is gray with a grayish-white streak, does not fluoresce under ultraviolet light, and has no observable cleavage or parting. Mohs hardness is 4-5, and pieczkaite is brittle with an irregular fracture. The calculated density is 3.783 g/cm3. Optical properties were measured using a Bloss spindle stage at a wavelength of 590 nm (using a gel filter). Pieczkaite is uniaxial (-) with indices of refraction ω = 1.696, ε = 1.692, both ±0.002. Pieczkaite is hexagonal, space group P63/m, a = 9.504(4), c = 6.347(3) Å, V = 496.5(1) Å3, Z = 2, c:a = 1:0.6678. The six strongest lines in the X‑ray powder diffraction pattern are as follows: d (Å), I, (hkl): 2.794, 100, (2̅31, 1̅31); 2.744, 88, (030); 2.639, 34, (1̅22); 2.514, 25, (031, 022); 1.853, 25, (3̅42, 1̅42); 3.174, 24, (002). Chemical analysis by electron microprobe gave P2O5 37.52, MnO 41.77, FeO 2.45, CaO 13.78, Cl 3.86, H2O 0.60, O≡Cl -0.87, sum 99.11 wt% where the H2O content was calculated as 1 - Cl apfu. The resulting empirical formula on the basis of 12 O anions is (Mn3.36Fe0.20Ca1.40)Σ4.96 (P1.01O4)3(Cl0.62OH0.38)1.00, and the end-member formula is Mn5(PO4)3Cl. The crystal structure of pieczkaite was refined to an R1 index of 4.07% based on 308 observed reflections collected on a three-circle rotating-anode diffractometer with MoKα X-radiation. Pieczkaite is isostructural with apatite, Mn is the dominant cation at both the [9]- and [7]-coordinated-cation sites in the structure, and Cl is the dominant monovalent anion.