Explain how index minerals allow a scientist to understand the history of a metamorphic rock?

Index minerals are defined as those minerals that signifies how much the original or source rock has undergone metamorphism. It indicates the grade of metamorphism. These index minerals are formed under very high pressure and temperature condition that is commonly found to be present in the metamorphic rocks.

Some of the examples of index minerals are kyanite, sillimanite, garnet, chlorite, staurolite, and epidote.

Thus, the index minerals determine the grade of metamorphism.

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."