Wopmayite, Ideally Ca6Na3 Mn(Po4)3(PO3Oh)4, A New Phosphate Mineral From The Tanco Mine, Bernic Lake, Manitoba: Description And Crystal Structure

Abstract

Wopmayite, ideally Ca 6 Na 3 □Mn(PO 4 ) 3 (PO 3 OH) 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 {1011} 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/cm 3 . It is uniaxial (-), ω = 1.617, ɛ = 1.613, both ± 0.002. Wopmayite is hexagonal-rhombohedral, space group R 3 c , a 10.3926(2), c 37.1694(9) A, V 3476.7(2) A 3 , Z = 6, a:c = 1:3.577. The seven strongest lines in the X-ray powder-diffraction pattern are as follows: d (A), I , ( h k l ): 2.858, 100, (02.10); 3.186, 88, (234); 2.589, 68, (240); 5.166, 33, (120); 6.421, 32, (114); 8.017, 31, (012); and 3.425, 29, (1110). Chemical analysis by electron microprobe gave P 2 O 5 46.40, Al 2 O 3 0.38, Fe 2 O 3 0.80, FeO 0.96, MnO 3.74, MgO 0.41, CaO 37.65, SrO 0.91, Na 2 O 5.43, and H 2 O(calc) 2.00, sum 98.68 wt.%. The H 2 O content was determined by crystal-structure analysis. On the basis of 28 O apfu , the empirical formula is (Ca 7 .19 Na 1.88 Sr 0.09 ) Σ9.16 (Mn 0.56 Mg 0.11 Fe 2+ 0.14 Fe 3+ 0.11 Al 0.08 ) Σ1.00 (PO 4 ) 4.63 (PO 3 OH) 2.37 , and the endmember formula is Ca 6 Na 3 □Mn(PO 4 ) 3 (PO 3 OH) 4 . The crystal structure of wopmayite was solved by direct methods and refined to an R 1 index of 2.21% based on 2288 unique observed reflections collected on a three-circle rotating-anode ( M o K α X-radiation) diffractometer equipped with multilayer optics and an APEX-II detector. Wopmayite has a structural unit consisting of an [M 2+ (PO 4 ) 6 ] arrangement that is topologically the same as the structural units in the whitlockite and merrillite structures. The [M 2+ (PO 4 ) 6 ] clusters are linked by Ca polyhedra and (PO 3 Φ) groups of the form {Ca 9 X (PO 3 Φ)} 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).