Precision and components of variance in the modal analysis of a coarse-grained augen gneiss

Point counting of large stained slabs under a microscope is recommended for rocks that are so coarse-grained as to require a very large number of thin sections per specimen for a moderately precise estimate of their mode, but not so coarse-grained as to make field identification of minerals a reliable procedure. The method is applied to a quartz-mangeritic augen gneiss, and the variability between the specimens is broken down into components of variance: The operator plus counting component is not significantly different from binomial expectation; the within-specimen component is used to predict the number of slabs required to obtain a mode of given precision; the between-specimen component is unknown, but a rough estimate of its maximum size is possible for designing sampling plans in similar rocks.