The modal mineral content of a rock is the relative ratio of the minerals occurring in it. It is usually expressed in percent by volume (% by volume), more rarely by weight (% by weight). The modal mineral population is a descriptive petrographic characteristic of a rock and is determined by microscopic counting of rock thinnings with the help of a point counter. An estimation can also be made by looking at a handpiece by means of a hand-held loupe in the field, whereby the examination should be carried out at a fresh fracture site without weathering influences. In contrast to the normative mineral stock, the modal mineral stock is a real property of a rock and refers to the minerals actually occurring in it.
For many types of rocks, particularly for coarse-grained magmatic rocks, but also for numerous rocks of metamorphic and sedimentary origin, the modal mineral stock is an important criterion for classification, Eg in the route diagram. Edit source text
wiki