The association qingheiite-beusite-lithiophilite in Santa Ana pegmatite, San Luis, Argentina

GALLISKI, M.A.; OYARZÁBAL, J.C.; MÁRQUEZ-ZAVALÍA, M.F.; CHAPMAN, R.

CANADIAN MINERALOGIST

MINERALOGICAL ASSOC CANADA

Lugar: Toronto, Canada; Año: 2009 vol. 47 p. 1213 - 1223

0008-4476

Qingheiite, the Mg end-member of the wyllieite group of transition-metal phosphates, whith formula Na2MnMgAl(PO)4 and known so far only from the type locality, was discovered in a new occurrence and association. It was found in the Santa Ana mine (32º53´32"S, 65º55´43"W), San Luis province, Argentina, in a beryl type, beryl-columbite-phosphate subtype of rare-element pegmatite, and in closely related La Empleada dyke. At Santa Ana, qingheiite forms irregular veinlets or patches of deep jade-green color, approximately 5 cm thick, in a nodule of beusite interlaminated with lithiphilite. The average empirical formula is (Na3.567 []0.421K0.010Ba0.002)4.000(Na2.000)2.000(Mn1.806Ca0.167Na0.027)2.000Mn4.000 (Mg2.890Fe0.515Mn0.595)4.000(Al1.870Fe3+0.950Zn0.030Fe2+1.150)4.000(PO4)11.919. Least squares refinement of X-ray powder-diffraction data of qingheiite gives a 11.884(2), b 12.459(2), c 6.439(1)A, B 114.31(1)º, V 867.6(2)A3. The host beusite has the formula (Mn1.734Fe0.666Ca0.394Mg0.150)2.944(PO4)2.010 and cell dimensions a 8.790(3), b 11.475(2), c 6.156(2)A, B 99.11(2)º, V 613.11(2)A3. Lithiophilite has the composition Li (Mn0.497Fe2+0.312Mg0.210)1.019(PO4)0.988, with a 6.063(2), b 10.324(6), c 4.712(2)A,  V 294.96 A3. The original primary phosphate of the nodule crystallized with a chemical composition of a Li-bearing beusite enriched in Mg, derived possibly by contamination by the host rock after the crystallization of the border and wall zones of the pegmatite. The exsolution of this precursor led to the formation of a lamellar intergrowth of beusite and lithiophilite enriched in Mg. Subsequent crystallization of qingheiite is attributed to veining, produced possibly by a late-stage, fluid-rich peraluminous melt