Essex County, New York
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