shonkinite - teaching hand/display specimen of an uncommon dark syenite
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The Mountain Pass shonkinite-syenite intrusions are part of an unusual complex of eight intrusive plugs ranging in composition from shonkinite to carbonatite, the primary ore of the recently reopened Mountain Pass rare-earth mine.
Syenite forms with the low degree of melting that occurs in a granitic parent rock in a subduction zone or in an area of thick continental crust. A low degree of melting releases potassium into the melt to produce orthoclase/microcline, with slower cooling producing microcline. A greater degree of melting would cause the release of calcium and sodium, the feldspar would become plagioclase, and a granite would form.
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