Isograd

An isograd is a concept used in the study of metamorphic rocks.

An isograd is a theoretical surface comprising points all at the same metamorphic grade, and thus separates metamorphic zones whose rocks contain different index minerals.

The minerals present in a metamorphic rock are important because laboratory experiments at high pressures and temperatures have provided a lot of information on the pressure and temperature conditions under which certain metamorphic minerals form.

For example, with increasing temperature and pressure the first minerals to form from a shale are micas, particularly chlorite and biotite.

With increasing temperature and pressure garnet appears, and then kyanite (at relatively high pressure) or sillimanite (at relatively high temperature).