Olivine metagabbro
Essex County, New York

Blocky plagioclase feldspar crystals are surrounded by pyroxene. The uniform color of the pyroxene indicates that it is a single crystal which grew around the feldspar as the original igneous rock crystallized from magma. Later, the rock was reheated and compressed, causing a complex chemical reaction that replaced parts of the feldspar with garnet. Notice how the garnet tends to follow the outlines of the feldspar grains. The original feldspar had zones of slightly different composition, and the garnet replaced only those zones with the right composition for the reaction to occur.

Crossed polarizers

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