Staining in petrographic thin sections

STAINING FELDSPARS

Houghton (1980) gave a method for staining plagioclase and alkali feldspar which is more reliable than previous recipes.

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.

J. Miller.

MODIFIED STAINING TECHNIQUE FOR STAINING CARBONATES IN THIN SECTION.

J.A.D.dickson

geo dept 

Queen Mary College London E.

     Carbonate minerals are stained over a set period of time with alizarin red s and potassium ferricyanide only if they will react with dilute hydrochloric acid solution, with which the stain is prepared. The rates of solution of carbonates in the acid control the intensity of color development. For calcite, the rate of solution varies with the optic orientation of the section. The speed of carbonate solution is changed if the acid concentration is altered, but only at concentrations of about 0.1N is the optic orientation of calcite differentiated by the stain. Etching reduces thin section thickness and clarifies rock texture.

    Staining with alizarin red-S differentiates carbonate minerals into two groups. Aragonite, calcite, witherite and cerussite, which dissolve rapidly in dilute hcl, are stained, while dolomite, siderite, magnesite and rhodochosite, which reacts much more slowly with the acid,  remain unstained. The distribution of ferrous iron, as distinguished by staining with potassium ferricynide, has proven to be highly significant in the genesis of cements. Ferrous iron can be in

 The distinction between dolomite and calcite, of which most ancient limestones are composed, is of considerable importance in limestone petrology. The modified staining technique described not only gives positive color differentiation of these two important carbonate minerals but also, because of their different solubility in hcl gives a difference of thickness in thin section.

oduced at any one stage in cementation, or repeatedly, forming zoned patterns. The paragenesis of zoned ferroan cements can be reconstructed after staining. Solution of the more soluble original constituents can some times be dated in relation to cementation. Ferroan calcite can be secondary in origin and is usually associated with replacement minerals.

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