The first or last appearance of an index mineral ( the place where a metamorphic reaction is observed ) forms an easily recognizable isograd.
12.
Thus, as with its other polymorphs, andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.
13.
This new mineral that is produced in the metamorphosed rock is the index mineral, which indicates the minimum pressure and temperature the protolith must have achieved in order for that mineral to form.
14.
Andalusite, kyanite, and sillimanite are naturally occurring aluminosilicate minerals that have the composition polymorphs is located at a temperature of 500 �C and a pressure of 0.4 GPa . These three minerals are commonly used as index minerals in metamorphic rocks.
