Methodology for determination of petrographic structures using the MIU-5M device

Abstract

The authors develop a statistical method for determining the structures of rocks. A mathematically rigorous definition of the petrographic structure through the probabilities of binary intergranular contacts is proposed. The petrographic structure is defined as an invariant aspect of rock organization algebraically expressed by the canonical diagonal form of symmetric matrix of contact probabilities and visualized by structural indicatrices - 2 order surfaces. A consistent nomenclature of petrographic structures has been proposed. Earlier the method showed itself well in describing the structures of ijolites and urtites of the Khibiny Mountains, as well as amphibolites of the islands of Keretsky Archipelago (the White Sea). Mass application of the method is restrained by the routine procedure of calculating the probabilities of intergranular contacts in petrographic thin sections under a microscope. The problem of modern optical structure analyzers is assignment different sections of the same anisotropic mineral to different minerals, and separation blocks of a mineral individual to different individuals. The paper describes the experience of using the domestic MIU-5M device for scanning petrographic thin sections by the linear method of Rosival. The combination of visual control and minimum automation of the process allows us to achieve convergence of the structural types with results obtained by manual processing of image of petrographic thin section. Additionally, thin sections have been studied, for which the structural types differ with various methods of counting. It has been revealed that the presence of poikilitic ingrowths and features of the mineral composition (the existence of hypergene changes or minerals with similar optical properties) affect the statistics of probabilities. Recommendations are given on the selection of the scanning step depending on the grain size in the studied rock.