Gazzi-Dickinson method

The Gazzi-Dickinson method is a point-counting technique used in geology to statistically measure the components of a sedimentary rock, chiefly sandstone.

The main focus (and most controversial) part of the technique is counting all sand-sized components as separate grains, regardless of what they are connected to.

On each randomly selected point that lands on a sand grain, the operator must determine the make-up of the area chosen, i.e. whether it is a mineral grain that is sand sized (larger than 62.5 micrometers) or a finer-grained fragment of another rock type, called a lithic fragment (e.g. a sand-sized piece of shale).

[2][3] Dickinson and his students (most notably Raymond Ingersoll, Steven Graham, and Chris Suczek)[4][5][6] at Stanford University in the 1970s established the method and its use to use the composition of sandstones to infer tectonic processes.

This was in contrast to ideas presented by sedimentary geologists at Indiana University at the time, who used the more traditional "QFR" or "rock fragment" method of Robert Folk (1974)[7] (which later grew into the Folk classification scheme), in which all grains that are connected are considered rock fragments, and the individual components are disregarded.

A sand grain that could be used for the Gazzi-Dickinson method. Scale box in millimeters, plane-polarized light on top, cross-polarized light on bottom. Landing on the large phenocryst of amphibole (center, clear in plane light, orange in cross-polarized light) would count as a mineral grain in the Gazzi-Dickinson method because it is sand-sized. Landing on the plagioclase -rich groundmass surrounding the grain would count as a volcanic lithic fragment. It would count as a volcanic rock fragment in the Folk Classification / QFR classification regardless of where the microscope landed in the point count.