Thin sections are widely used to characterize the size of grains in igneous and metamorphic rocks. The 2D to 3D conversion of the grain size distributions in not a simple task: many researchers discussed the problem during few last decades. One of the most common way to resolve the problem is the Schwarz-Saltykov back-substitution method. There was a long-lasting restriction for application of this general-purpose stereological technique as usable essentially in the specific case of spherical grains. Actually, the computer technologies has extend its approach to the non-spherical grains also, but only with shapes which intersection probability distributions are derived numerically. The obtained results have put the question of stereological approximation accuracy. From this point of view, there are discussed the data on stereological conversion for simulated aggregates of spheres with normal and log-normal size distributions and for natural aggregates of metamorphic garnet. Furthermore, the procedure to calculate the intersection probability distributions for rectangular parallelepipeds with specified aspect ratios is treated, and the data on stereological conversion for simulated aggregates of prisms are reviewed. For an example, that developed method is applied to determine the actual size distributions of plagioclase crystals observed in some basaltic rocks.
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