Crystal chemistry of wyllieite-type phosphates

HATERT, F. FRANSOLET, A.M. RODA-ROBLES, E. GALLISKI, M.A.

Estambul

Congreso; 25th European Cristallographic Meeting; 2009

European Cristallographic Association

The wyllieite group of minerals consists of Na-Mn-Fe-Al-bearing phosphates which exhibit a crystal structure topologically similar to the alluaudite structure. However, the ordering of cations in the wyllieite structure induces a splitting of the M(2) and X(1) sites of alluaudite into the M(2a) M(2b) and X(1a) X(1b) positions. Consequently, the C2/c space group of alluaudite transforms into P21/n in wyllieite, with no significant change of the unit-cell parameters, and with a structural formula which corresponds to X(2)X(1a)X(1b)M(1)M(2a)M(2b)(PO4)3.In granitic pegmatites, wyllieite-type phosphates display chemical compositions ranging from Na2(Mn,Fe2+)Fe2+Al(PO4)3 to Na(Mn,Fe2+)Fe3+Al(PO4)3, with Ca2+ or Mn2+ replacing Na+ on the X(2), X(1a) and X(1b) sites, Mg2+ replacing iron on the M(2a) site, and Mg2+ or Fe3+ replacing Al3+ on the M(2b) site, where represents a lattice vacancy. The name wyllieite corresponds to Na2MnFe2+Al(PO4)3, while the name rosemaryite designates the more oxidized compositions, such as NaMnFe3+Al(PO4)3. The prefix ferro- is then added if Fe2+ dominates in the M(1) site, thus leading to ferrowyllieite, Na2Fe2+2Al(PO4)3, and to ferrorosemaryite, NaFe2+Fe3+Al(PO4)3. The name qingheiite has been introduced for the Mg-rich equivalent of wyllieite, Na2MnMgAl(PO4)3.Single-crystal structure refinements of ferrorosemaryite from the Rubindi pegmatite, Rwanda (R1 = 2.43 %, a = 11.838(1), b = 12.347(1), c = 6.2973(6) Å, β = 114.353(6)°), of rosemaryite from the Buranga pegmatite, Rwanda (R1 = 4.01 %, a = 12.001(2), b = 12.396(1), c = 6.329(1) Å, β = 114.48(1)°), of wyllieite from the Buranga pegmatite (R1 = 2.74 %, a = 11.954(2), b = 12.439(2), c = 6.406(1) Å, β = 114.54(1)°), and of qingheiite from the Santa Ana pegmatite, Argentina (R1 = 2.65 %, a = 11.878(3), b = 12.448(2), c = 6.438(2) Å, β = 114.49(1)°), indicate that Al is predominant on the M(2a) site, not on the M(2b) site as observed in ferrowyllieite. The morphologies of the X(1a) and X(1b) crystallographic sites correspond to a distorted octahedron and to a distorted cube, respectively. The [7+1]-coordinated X(2) site of rosemaryite is a very distorted gable disphenoid, similar to the A(2)’ site of the alluaudite structure.The structural features of rosemaryite, ferrorosemaryite, wyllieite, and qingheiite are compared to those of other natural and synthetic wyllieite-type phosphates, and the role played by Al to stabilize the wyllieite structure is discussed in detail.