Optical mineralogy (petrographic microscopy) 



Optical mineralogy utilizes the theory and mineral specimens of identical gross chemical 



methods of optical crystallography for the composition, but with different thermal his- 



description, classification, and identification tories, commonly show noteworthy differ- 



of opaque and nonopaque minerals, espe- ences in their optical properties, 



cially by observations under the polarizing Very few minerals are "pure" in the sense 



microscope. Prior to about 1900 the polariz- that the chemical composition can be ex- 



ing microscope was used principally for the pressed by a formula in which the ratios of 



determination of the texture and mineralogy the numbers of the atoms can be expressed 



of rocks, and emphasis was placed on the in simple, whole numbers. Isomorphism is 



identification of minerals by properties that common in most minerals and comes about 



could be measured in thin sections mounted from mutual substitution (diadochy) of 



on glass slides. Since 1900 instrumentation atoms or groups of atoms in crystal struc- 



and techniques have improved rapidly, and tures. For example, monoclinic amphiboles 



examination of minerals under the polarizing have a generalized formula which can be 



microscope by a variety of methods has be- expressed as WsCX, Y)5Z8022(OH, F)2 , in 



come routine procedure in many fields of which W = Ca and Na; X = Mg, Fe", and 



science and technology. Mn; Y = Al and Fe'"; and Z = Si and Ti. 



An ultimate purpose of optical mineralogy Each element contributes more or less to the 

 is the accurate correlation of optical proper- optical and other properties depending on 

 ties of all minerals with crystal structure, its relative amount in a crystal, the proper- 

 chemical composition, and other physical ties of the individual atoms, and their posi- 

 properties, and to arrive at an understanding tions and manner of coordination in the 

 of the influence of various physicochemical crystal structure. 



factors on the optical properties. With such Correlation of optical properties with 



information the optical technique is a valu- chemical composition and physical state 



able tool for identification of mineral sub- differs in complexity from one mineral series 



stances, estimation of chemical composition, to another. Quartz, for example, has a 



and determination of the natural history of crystal structure that, within ordinary tem- 



crystallization. Optical studies, together with perature ranges, is incapable of accepting 



x-ray examinations and crystallochemical in- foreign atoms except in trace amounts. Ac- 



vestigations within a large range of pres- cordingly, the optical properties of quartz 



sures and temperatures provide a complete are constant from specimen to specimen, and 



and powerful approach to the study of the are diagnostic. Minerals in which isomor- 



nature and genesis of minerals. phism (diadochic substitution) is character- 



The physical properties of a mineral in istic show wide variations in optical proper- 

 equilibrium with its environment are con- ties. Some appreciation of the contributions 

 stant and correlate directly with the chemi- of individual elements or radicals to optical 

 cal composition. The physical properties of a properties is gained from a study of the spe- 

 mineral of the same chemical composition cific refractive energies (Jaffe, 1956) or 

 which is not in equilibrium differ from those specific refractive capacities (Allen, 1956), 

 of the equilibrium mineral to an extent de- particularly as the amounts of the elements 

 pending on the departure of the atomic con- or radicals in crystals influence the refractive 

 figuration of the crystal structure from that indices, 

 for the equilibrium state. For example, two A convenient method for showing how op- 



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