Abstract

Wopmayite, ideally Ca6Na3□Mn(PO4)3(PO3OH)4, is a new secondary mineral from the Tanco mine, Bernic Lake, Manitoba. It occurs in vugs in a single 5–10 cm mass of phosphate-carbonate mineralization in a spodumene-rich boulder found in the dumps of the Tanco Mine, Bernic Lake, Manitoba, Canada. It is a secondary mineral that crystallized together with rhodochrosite, quartz, whitlockite, apatite, and other phases after dissolution of primary lithiophosphate by hydrothermal solutions. The initial crystal of wopmayite was a corroded {101̅1} rhomb ∼150 microns across. Wopmayite is colorless to white to pale pink with a white streak and a vitreous luster, and does not fluoresce under ultraviolet light. It has a Mohs hardness of 5, is brittle, has an irregular to subconchoidal fracture, and shows no cleavage or parting. The calculated density is 3.027 g/cm3. It is uniaxial (-), ω = 1.617, ɛ = 1.613, both ± 0.002. Wopmayite is hexagonal-rhombohedral, space group R3c, a 10.3926(2), c 37.1694(9) Å, V 3476.7(2) Å3, Z = 6, a:c = 1:3.577. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: d (Å), I, (h k l): 2.858, 100, (02.10); 3.186, 88, (2̅34); 2.589, 68, (2̅40); 5.166, 33, (1̅20); 6.421, 32, (1̅14); 8.017, 31, (012); and 3.425, 29, (1̅110). Chemical analysis by electron microprobe gave P2O5 46.40, Al2O3 0.38, Fe2O3 0.80, FeO 0.96, MnO 3.74, MgO 0.41, CaO 37.65, SrO 0.91, Na2O 5.43, and H2O(calc) 2.00, sum 98.68 wt.%. The H2O content was determined by crystal-structure analysis. On the basis of 28 O apfu, the empirical formula is (Ca7.19 Na1.88 Sr0.09)Σ9.16(Mn0.56 Mg0.11Fe2+0.14 Fe3+0.11Al0.08)Σ1.00(PO4)4.63(PO3OH)2.37, and the endmember formula is Ca6Na3□Mn(PO4)3(PO3OH)4.

The crystal structure of wopmayite was solved by direct methods and refined to an R1 index of 2.21% based on 2288 unique observed reflections collected on a three-circle rotating-anode (MoKα X-radiation) diffractometer equipped with multilayer optics and an APEX-II detector. Wopmayite has a structural unit consisting of an [M2+(PO4)6] arrangement that is topologically the same as the structural units in the whitlockite and merrillite structures. The [M2+(PO4)6] clusters are linked by Ca polyhedra and (PO3Φ) groups of the form {Ca9X(PO3Φ)} where Φ = O,OH and X = (□, Na, Ca), depending on the mineral species. Wopmayite is related to whitlockite by the substitution Na + H → Ca + □, whereby Na is incorporated primarily at the Ca(3) site and H attaches to the P(1) tetrahedron to produce an acid-phosphate group. Thus merrillite contains no acid-phosphate group, whitlockite contains a single acid-phosphate group at P(3), and wopmayite contains acid-phosphate groups at both P(1) and P(3).

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