New data on hemihedrite from Arizona

Abstract

Hemihedrite from the Florence Lead-Silver mine in Pinal County, Arizona, USA was first described and assigned the ideal chemical formula Pb10Zn(CrO4)(6)(SiO4)(2)F-2, based upon a variety of chemical and crystal-structure analyses. The primary methods used to determine the fluorine content for hemihedrite were colorimetry, which resulted in values of F that were too high and inconsistent with the structural data, and infrared (IR) spectroscopic analysis that failed to detect OH or H2O. Our reinvestigation using electron microprobe analysis of the type material, and additional samples from the type locality, the Rat Tail claim, Arizona, and Nevada, reveals the absence of fluorine, while the presence of OH is confirmed by Raman spectroscopy. These findings suggest that the colorimetric determination of fluorine in the original description of hemihedrite probably misidentified F due to the interferences from PO4 and SO4, both found in our chemical analyses. As a consequence of these results, the study presented here proposes a redefinition of the chemical composition of hemihedrite to the ideal chemical formula Pb10Zn(CrO4)(6)(SiO4)(2)(OH)(2). Hemihedrite is isotypic with iranite with substitution of Zn for Cu, and raygrantite with substitution of Cr for S. Structural data from a sample from the Rat Tail claim, Arizona, indicate that hemihedrite is triclinic in space group P (1) over bar, a = 9.4891(7), b = 11.4242(8), c = 10.8155(7) angstrom, alpha = 120.368(2)degrees, ss = 92.017(3)degrees, gamma = 55.857(2)degrees, V = 784.88(9) angstrom(3), Z = 1, consistent with previous investigations. The structure was refined from single-crystal X-ray diffraction data to R-1 = 0.022 for 5705 unique observed reflections, and the ideal chemical formula Pb10Zn(CrO4)(6)(SiO4)(2)(OH)(2) was assumed during the refinement. Electron microprobe analyses of this sample yielded the empirical chemical formula Pb-10.05(Zn0.91Mg0.02)(Sigma) (= 0.93) (Cr5.98S0.01P0.01)(Sigma = 6.00) Si1.97O34 H-2.16 based on 34 O atoms and six (Cr + S + P) per unit cell.