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syenite - hand/display specimen of a very dark syenite - melasyenite

$ 9.50

Syenite is an uncommon coarse-grained intrusive igneous rock with the same general composition as granite, but with less than 5% quartz. The feldspar, dark pink to purple in this syenite, is microcline. Hornblende, aegirine, phlogopite and biotite occur in varying amounts, giving the outcrop a maroon cast and making the syenite darker than usual. Melasyenite, from the Greek melas for black, refers to the dark shade of this syenite.

This syenite occurs in the Mescal Range in California, at Mountain Pass, where two composite syenite-shonkinite bodies are associated with the Mountain Pass carbonatite complex that contains the ore that is being mined for rare earth elements by MP Mine Operations. Melasyenite and shonkinite are both unusual igneous rocks.

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.

The specimen with a pencil and one coin is cut flat on one side and is standing on that side in the photo. Good for a display.




Select a specimen: You can select a specimen by telling us what is in the photo with it, a blue or black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.

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 We usually have this rock type in hand specimens also.  

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