Sanidinite Facies

sanidinite facies, one of the major divisions of the mineral facies classification of metamorphic rocks, the rocks of which form under the most intense conditions of contact metamorphism—either at the contacts of igneous intrusions with the surrounding rocks or in inclusions of other rocks in intrusions. Temperatures approaching the melting point of the rock may be reached; the pressure is relatively low and depends upon the depth of the intrusion. More intense conditions cause fusion, and less intense conditions form rocks of the hornfels facies. Minerals characteristic of such rocks include sanidine, corundum, cordierite, and sillimanite; glass from partial fusion or relict minerals from less intense temperatures may also be present. The particular mineralogy formed depends largely upon the composition of the parent rock surrounding the igneous intrusion. Volatiles such as carbon dioxide and water are expelled from the rock during the mineral-forming reactions.