MODIFIED STAINING TECHNIQUE FOR STAINING CARBONATES IN THIN SECTION.

J.A.D.dickson

 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

    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 introduced 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.

 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.