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

Abstract

Qingheiite, the Mg end-member of the wyllieite group of transition-metal phosphates, with a formula Na2MnMgAl(PO4)(3) 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 (32 degrees 53'32"S, 65 degrees 55'43"W), San Luis province, Argentina, in a beryl type, beryl-columbite-phosphate subtype of rare-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, similar to 0.5 cm thick, in a nodule of beusite interlaminated with lithiophilite. The average empirical formula is (Na-3.567 square 0.421K0.010Ba0.002)(Sigma 4.000) (Na-2.000)(Sigma 2.000) (Mn1.806Ca0.167Na0.027)(Sigma 2.000) Mn-4.000 (Mg2.890Fe0.515Mn0.595)(Sigma 4.000) (Al1.870Fe0.9503+Zn0.030Fe1.1502+)(Sigma 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) angstrom, beta 114.31(1)degrees, V = 867.6(2) angstrom(3). The host beusite has the formula (Mn-1.734 Fe-0.666 Ca-0.394 Mg-0.150)(Sigma 2.944) (PO4)(2.010) and cell dimensions a 8.790(3), b 11.475(2), c 6.156(2) angstrom, beta 99.11(2)degrees, V = 613.11(2) angstrom(3). Lithiophilite has the composition Li (Mn0.497Fe0.3122+Mg0.210)(Sigma 1.019) (PO4)(0.988), with a 6.063(2), b 10.324(6), c 4.712(2) angstrom, V = 294.96 angstrom(3). 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.