OPTICAL MINERALOGY 



tical properties vary with composition is 

 afforded by plotting data of various kinds 

 on charts. In simpler mineral series, such as 

 the olivine series, [(Alg, Fe)2SL04], the com- 

 position of a particular member of the series 

 can be expressed in weight or molecular pro- 

 portions or percentages of Mg2Si04 and 

 Fe2Si04 . Using Mg2Si04 and Fe2Si04 as 

 "end members" simple diagrams can be pre- 

 pared which show how the optical properties 

 vary with composition. Mineral series for 

 which compositions can be expressed in 

 terms of three "end members" are plotted 

 on triangular diagrams, and so on. However, 

 mineral series with more than four end mem- 

 bers refiuire polyhedral plots, and the diffi- 

 culty of plotting data and interpreting 

 diagrams becomes very great, if not almost 

 insurmountable. In complex mineral series 

 an effort generally is made to group similar 

 end members in such a way that data can 

 be plotted on simple diagrams, and it is un- 

 derstood that compositions determined from 

 optical data plotted on the diagrams are only 

 approximations. 



Laboratory investigations in optical min- 

 eralogy are of three general kinds: (1) study 

 of crystals or fragments in liquid immersions, 

 (2) examination of single crystals or aggre- 

 gates in petrographic thin sections ground to 

 a thickness of 0.03 mm and mounted on a 

 glass slide, and (3) examination of opaque 

 substances in polished sections by reflected 

 light, together with systematic etching and 

 microchemical analysis. All techniques em- 

 ploy the polarizing microscope as more or 

 less modified for a specific purpose. The 

 theory and practice of the immersion method 

 (Wahlstrom, 1960), the thin-section method 

 (Moorehouse, 1959), and the polished-sec- 

 tion method (Short, 1940, and Freund, 1954) 

 are reviewed in several books and in numer- 

 ous articles in chemical, mineralogical, and 

 geological journals. Summaries of properties 

 of nonopaque minerals (Winchell and Win- 

 chell, 1951; Larsen and Berman, 1934) and 



of opaque minerals (Short, 1940) are found 

 in standard referances and handboods. 



The fundamental optical properties of 

 nonopacjue minerals are the principal re- 

 fractive indices measured for one or more 

 reference wave lengths of light, the crystal- 

 lographic orientation of the vibration direc- 

 tions of light corresponding to each of the 

 principal refractive indices, and the amount 

 and manner of absorption of light for various 

 directions of vibration and transmission of 

 light by a crystal. All other optical prop- 

 erties can be calculated or predicted if the 

 fundamental properties are known. 



The immersion technique permits accur- 

 ate measurement of refractive indices by 

 comparison of the mineral with liquids of 

 known refractive indices, observation of the 

 manner of absorption of transmitted light, 

 and, under favorable circumstances, de- 

 termination of optic orientation. Other 

 properties can be calculated or can be meas- 

 ured with special optical accessories. In 

 special applications of the immersion tech- 

 nique crystals or fragments are observed on 

 special multiaxis auxiliary stages that per- 

 mit rotations of preparations into any de- 

 sired position. Valuable additional equip- 

 ment is a monochromator for varying the 

 wave length of the source of illumination, 

 and temperature control apparatus. 



In the thin-section method refractive in- 

 dices are not measured directly, but optic 

 orientation and differential absorption of 

 light can be determined by direct observa- 

 tion. Other diagnostic properties are noted 

 or measured with the aid of a variety of op- 

 tical measuring devices. As in the immersion 

 method a multiaxis stage assists in many 

 measurements. 



Polished sections of opaque minerals are 

 examined in reflected light from a vertical 

 illuminator attachment. Various optical ef- 

 fects are noted, and properties such as re- 

 flectivity, color, and rotation of polarized 

 light may be estimated visually or measured 

 with spectrophotometric equipment. In prac- 



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