petrographic microscope. The fundamental advantage of the stage is that a thin 

 section can be oriented to almost any position. In addition to standard petro- 

 graphic equipment, only a 3- or 4-axis stage and a net for plotting are necessary 

 to carry on this work (fig. 4-5). Some suggestions for application of the 

 universal stage to sedimentary petrography and a bibliography of the general 

 technique appear in a paper by Gilbert and Turner (1949). Some uses of the 

 universal stage which are applicable to subsurface petroleum geology are dis- 

 cussed below. 



Mineral Identification 



The universal stage has several advantages over the normal thin-section 

 method of mineral identification. With the universal stage, one can determine 

 the relationships between optical and physicial properties. With only the micro- 

 scope, it is often impossible to find grains oriented so that all optical properties 

 can be determined. With the stage, it is possible to tilt the section so that most 

 and often all the properties of a single grain can be determined. In Gilbert and 

 Turner's paper, a method is outlined for distinguishing some of the carbonate 

 minerals in thin section. The universal stage is a powerful easy-to-use tool that 

 should not be neglected in detailed rock analyses. 



Miscellaneous Textural Studies 



Because it is possible to orient a thin section with the universal stage, many 

 textural features otherwise unobserved become visible. Grain contacts can be 

 studied to ascertain whether they are normal depositional, intergrown, or other- 

 wise interfered. Whether grain angularity is due to abrasion or to authigenic 

 outgrowths can be determined because outgrowths are less difficult to recognize 

 with the stage. Where multiple cements are present, it is often possible to deter- 

 mine the order of deposition. Many cementing materials develop crystal faces 

 during deposition. These faces are frequently preserved when a subsequent 

 cement is deposited. By determining relations between the optical properties and 

 the crystal faces, it is possible to establish to which mineral the face belongs, 

 thereby establishing a genetic relationship. This method is described by Gilbert 

 and Turner (1949). 



Petrofabric Analysis 



One of the most important uses of the universal stage involves petrofabric 

 analysis, or determination of preferred orientation of grains. When rocks are 

 formed in, or subsequently subjected to, an anisotropic environment — one that 

 does not have the same properties in all directions — there is a tendency for the 

 rocks to reflect the anisotropy. This condition is more conspicuous in some rocks 

 than in others; and in many environments, the anisotropy is too weak to induce 



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