A rapid method for hand picking potassium-rich feldspar from silicic tephra

We detail a method for hand picking potassium-rich feldspars from silicic tephra. The method, based on the well known sodium cobaltinitrite staining technique, is applicable to any sample where K content of feldspar exceeds ∼2 wt%. The method is well suited to mineral grains >0.25 mm and useful for grains as small as ∼0.125 mm. Alkali feldspar, plagioclase, and quartz are easily distinguishable after staining. Rinsing the stained feldspars along with ultrasonic treatment in distilled water and nitric acid removes the stain. 40Ar/39Ar analysis of sanidine separated by staining yields precise age estimates which are in excellent agreement with published ages for the youngest (2.06 Ma) Cerro Galán ignimbrite, NW Argentina.