Methods for separation and total stable isotope analysis of alunite

Abstract

Techniques are presented for the separation of alunite [KAl3(SO4)2(OH)<s] from commonly associated minerals and subsequent stable isotope analyses for 8D, 8 OoH 8 18Oso4 and 834S. These analyses can be performed successfully on most natural samples containing at least 100 mg of alunite. Associated minerals (typically quartz, kaolinite, and pyrite) are first minimized by hand picking or microdrilling. Ultrasonic suspension followed by elutriation is usually sufficient to remove clays. Other silicates and quartz are removed by HF digestion. Pyrite is removed by heavy liquid separation or by CrCl2-HCl digestion. For 8^8OsQ4 and S^S analyses, alunite is selectively dissolved in a heated solution of 0.5N NaOH. The filtered solution is titrated with HC1; sulfate is precipitated as BaSO4 which is subsequently analyzed for 834 S and 8 18Oso4 by conventional procedures. 8D analyses are performed on water derived from alunite by inductively coupled stepwise thermal decomposition that minimizes SO2 and FfeSCU production and formation of KNa oxides. 818OoH analyses are performed using a total fluorination method modified from that of Bulk oxygen produced by fluorination of alunite (818OfirF5), consists of 501% of the O2 from the SO4 site and 100% from the OH site. A large fractionation during fluorination of sulfates (A 18Oso4(GR-Fl)) is constant over the entire natural range of 8 18OsO4 values and is dependent on the ratio of K+Na to total oxygen. A18OsO4(GR-Fl)alunite (10.5%o in this laboratory) was determined by comparison of 818Oso4 values obtained by conventional graphite-reduction (GR) with those by fluorination (Fl) of reagent mixtures. 818OoH is calculated from a mass balance equation which includes terms for 818OfirF5, 818Oso4 (measured by graphite-reduction), A18Oso4(GR-Fl)alunite and the molar ratios of liberated OH and SO* oxygen. The calculated 818OoH is insensitive to uncertainties in O2 yield and generally reflects the total analytical uncertainty of the fluorination and sulfate graphitereduction technique.