Rocks petrographic classification

Image analysis of stained slabs (Duncan 1999) indicates these rocks should probably be referred to as ( ) augite syenite, ( ) hornblende quartz syenite, and ( V) biotite syenogranite (from mean petrographic classification Fig. 2). 

The relation between chemistry and petrography, the science of the classification of rocks, was somewhat closer since chemical composition was thought by some petrographers to offer a basis for rational classification. But early efforts to categorize rocks by their chemical composition were frustrated as it was discovered that it was impossible to find simple chemical formulas for these bodies. Rocks of different mineral composition fell into the same chemical categories, and rocks that were chemically alike were sometimes quite different in structure, texture, and field relations. Moreover, the success of H. Clifton Sorby s technique for the microscopic study of thin sections of rocks accentuated the tendency to identify rocks on the basis of external rather than internal properties (14, 15). 

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