Virginia R. Marshall

An Easy Field Test for Distinguishing Sodium from Potassium Water-Soluble Minerals

ith a similar appearance and depositional environment, W halite (NaCl) and sylvite (KCl) are often difficult to distinguish in the field. Both crystallize in the cubic crystal system (although their habits may vary-cg., cubic for halite versus octahedral for sylvite), both are colorless (or tinted various hues), and both taste salty (sylvite is more astringent, but the “taste test” is frequently ambiguous when the mineral is tainted with impurities). Identification can be particularly confusing when the two minerals crystallize together (Gaines et al. 1997). In association with our work with the American Chemical Society (Marshall 2000), we have developed a field procedure for distinguishing halite from sylvite. This test depends upon the solubility difference of sodium tetraphenylborate, (C,H,),BNa, and potassium tetraphenylborate, (C,H,),BK: the former is very soluble in water, whereas the latter is insoluble. The field procedure consists of washing the test sample (with distilled or deionized water, 2-10 milliliters, using an eyedropper) into a clean vial or beaker, then adding three drops of 1 percent sodium tetraphenylborate solution. If the test sample is halite (only sodium ions), then the washing remains clear, being composed of soluble sodium chloride and soluble sodium tetraphenylborate (fig. 1). If the test sample contains sylvite (potassium ions), then the washing immediately becomes cloudy with the precipitate of insoluble potassium tetraphenylborate-i.e., (C,H,),BNa + KC1 + (C,H,),BK + NaC1. This test only requires a trace of salt to be dissolved; hence, the washing of the test sample dissolves only a minute amount of the specimen and does not significantly alter its appearance. This convenient test requires a kit composed of only a wash bottle of distilled water, a clean receptacle, and a dropping bottle of the test reagent, which can be prepared by the tester.” As an example of the usefulness of this pethod, we tested a 20-cm muddy reddish sample from Eddy County, New Mexico, and demonstrated a differentiated mixture, with potassium and sodium minerals at different specific sites about the sample (fig. 2).