The original igneous rock, a gabbro, contained only olivine and feldspar. As pressure and temperature changed during metamorphism, the olivine and feldspar reacted to form layers of pyroxene and garnet. The pyroxene and garnet surround the olivine grains with coronas or halos of concentric rings. Here you see only part of a corona because of the high magnification. The feldspar at the bottom appears dark because it contains millions of tiny grains of spinel that replaced part of the feldspar as the corona formed. Reactions like this one occur very slowly. This corona was preserved when the rock became too cool for the reaction to continue.

Crossed polarizers