reaction rim
Quick Summary
A reaction rim is a mineral zone that forms around a larger crystal. It develops when a solidified mineral reacts with the surrounding liquid magma. This process is crucial in understanding mineral associations and phase equilibrium in rocks.12
In the context of mineralogy and petrology, reaction rims indicate that minerals were not in equilibrium when the rock formed. For instance, if olivine crystals are surrounded by a glass that is saturated or supersaturated with minerals, the olivine's outer edge might be replaced by a magnesium-rich pyroxene, forming a reaction rim. This happens when the first-crystallized mineral, olivine, begins to react with the magma to form a more stable mineral.23
The presence or absence of reaction rims helps petrologists determine if minerals in a rock were in equilibrium at the time of formation. If no reaction rims are visible between touching minerals, it suggests they were in equilibrium. Conversely, visible rims indicate that reactions occurred, signifying a lack of equilibrium.2
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