in source rock. When one is working with the restricted mineral suites of the 

 older rocks, it is important that the varieties of each mineral be recorded 

 because these subspecies may yield more information than the total suite. 

 Krynine (1946) demonstrates the stratigraphic use of the varietal properties of 

 tourmaline. He recognizes many subspecies of this mineral and suggests 5 

 different source areas based upon these variants. Detailed studies of other 

 resistant minerals may lead to similar results. 



The use of heavy minerals in correlating and identifying stratigraphic units 

 is described by Feo-Codecido (1956). In areas where paleontological data are 

 sparse or non-diagnostic, heavy-mineral studies may aid greatly in correlations, 

 as has been proved in Venezuela. It is general practice to define suites of 

 minerals for various stratigraphic units much as fossil assemblages are establish- 

 ed. Zones may be erected on the basis of mineral suites, relative abundance 

 or ratio of mineral species, and depositional sequence relationships. 



Heavy-mineral studies must be based on systematic sampling which is 

 necessary to establish the stratigraphic relationships. Conclusions based upon 

 the study of only a few scattered samples may be seriously in error. The petro- 

 grapher must analyze and evaluate in terms of thousands of concentrates — 

 not just a few hundred. 



STAIN METHODS Staining methods are used to distinguish cal- 



cite from dolomite, calcite from aragonite, 

 and quartz from feldspars. These methods have also proved helpful in identify- 

 ing certain clay minerals. The principal use of staining procedures is to 

 evaluate the textures and mineral distribution in a rock; however since these 

 methods are not precise, petrographic, X-ray, and chemical analyses should be 

 made if more exact mineralogic determinations are desired. 



Stain tests may be made on polished slabs, thin sections, or grain aggre- 

 gates. If textural relations are desired, a polished slab is preferred. Uniformity 

 in strength of solutions and immersion time is necessary for consistent and 

 reliable results. 



Colette and Dolomite 



Several methods may be used to distinguish calcite from dolomite. The most 

 widely used are the Fairbanks' (1925) method; the copper nitrate method, 

 first suggested by Mahler (1906) ; and the silver chromate method developed 

 by Lemberg (1892). Rodgers (1940) gives a thorough review of staining 

 methods as applied to calcite and dolomite. 



Fairbanks Method 



This method is a modification of one described by Lemberg (1887). The 

 stain solution is prepared by mixing 0.24 grams of haematoxylin, 1.6 grams of 



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