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Another specimen

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Typical Minerals
Na/Ca PLAGIOCLASE & MAFICS (amphibole)
about 50/50
Quartz - absent to trace
     An intermediate igneous rock from the middle of Bowen's Reaction Series. Typically intermediate colored with a mixture of light colored sodium plagioclase and dark colored amphibole (although biotite may also be present in traces). Appearance is often described as "salt and pepper" because of the mix. The mafic/plagioclase mix looks heavy on the mafic side in this specimen, but in fact is probably not far from 50/50. Our eyes see dark easier than light and so it looks darker than it is. Take a small portion and examine it carefully and you will see that black and white are about equal.
     Quartz is sometime present in diorites, but this specimen is probably too high in Bowen's Reaction series to have much if any.
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Tectonic Association
      Diorite is the result of fractional melting of a mafic parent rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building (subduction along the edge of a continent, such as with the Andes Mountains).
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