THE ASSOCIATION QINGHEIITE-BEUSITE-LITHIOPHILITE IN THE SANTA ANA PEGMATITE, SAN LUIS, ARGENTINA

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

CANADIAN MINERALOGIST

MINERALOGICAL ASSOC CANADA

Lugar: Ottawa; Año: 2009 vol. 47 p. 1213 - 1223

0008-4476

Qingheiite, the Mg end-member of the wyllieite group of transition-metal phosphates, with a formula Na2MnMgAl(PO4)3 and known so far from only the type locality, was discovered in a new occurrence and association. It was found in the Santa Ana mine (32°53’32” S, 65°55’43” W), San Luis province, Argentina, in a beryl type, beryl–columbite–phosphate subtype of arare-element pegmatite, and in the closely related La Empleada dike. At Santa Ana, qingheiite forms irregular granular veinlets or patches of deep jade-green color, ~0.5 cm thick, in a nodule of beusite interlaminated with lithiophilite. The average empirical formula is (Na3.5670.421K0.010Ba0.002)S4.000 (Na2.000)S2.000 (Mn1.806Ca0.167Na0.027)S2.000 Mn4.000 (Mg2.890Fe0.515Mn0.595)S4.000(Al1.870Fe3+0.950Zn0.030Fe2+1.150)S4.000 (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) Å, b 114.31(1)°, V = 867.6(2) Å3. The host beusite has the formula (Mn1.734 Fe0.666 Ca0.394Mg0.150)S2.944 (PO4)2.010 and cell dimensions a 8.790(3), b 11.475(2), c 6.156(2) Å, b 99.11(2)%, V = 613.11(2) Å3. Lithiophilite has the composition Li (Mn0.497Fe2+0.312Mg0.210)S1.019 (PO4)0.988, with a 6.063(2), b 10.324(6), c 4.712(2) Å, V = 294.96 Å3.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. Subsequentcrystallization of qingheiite is attributed to veining, produced possibly by a late-stage, fluid-rich peraluminous melt.