STAINING FELDSPARS

Houghton (1980) 

REAGENTS: (a) potassium rhodizonate (0.01g K-rhodizonate in 30ml distilled water. (b) Sodium cobaltinitrite (saturated solution; about50g per 100ml distilled water). (c) Barium chloride

(5% solution in distilled water).

PROCEDURE:

(a) Etch over hf vapor (55% hf solution) for 25-35 sec WARNING!! HYDROFLOURIC ACID IS SUPER CAUSTIC, LET THE PROS HANDLE IT.

(b) Remove the slide from etching box and drop into beaker with Na-cobaltinitrite. leave for 45 sec.

(c) Rinse slide in distilled water, blot dry or air dry. (d) Dip slide into beaker with BaCL2 solution for no more than 5 sec. (e) Dip in distilled water and agitate for 10 sec. (f) Place several drops of the rhodizonate solution on the damp surface leave until plagioclase grains become pink then rinse with water.(g) Air dry

The intensity of the pink plagioclase stain is proportional to the amount of calcium in the molecule: Albite/oligoclase will stain lighter than a more calcic plagioclase. Pure Na-albite will not take up any of the rhodizonate stain. Alkali feldspars are stained greenish-yellow. The accuracy of the stainsdecreases according to grain size: with finer grained specimens, the pink stain tends to pervade the surface and obscure the quartz grains.CATHODOLUMINESCENCE is a much better way of detecting feldspars in fine sands and siltstones.