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shonkinite - teaching hand/display specimen of an uncommon dark syenite

$ 9.50

Shonkinite is an uncommon dark syenite, with greater than 50% mafic minerals, where syenite normally is less than 30% mafic. This shonkinite is composed largely of phlogopite mica, originally recognized as biotite, with interstitial pink to purplish microcline, some olivine, diopside and amphibole. Mafic refers to minerals/igneous rocks rich in iron and magnesium. Relatively high density and a dark color are characteristics. This was collected just south of Mountain Pass, California, in the Mescal Range. The white flakes are actually reflections from the black phlogopite.

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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