Igneous Rocks

greenspun.com : LUSENET : Introductory Geology, Oswego State : One Thread

If early formed crystals are removed for magma, then why will the compostion of the remaining magma be different from the original formed compositon??

-- Vanessa Azevedo (nessi1024@aol.com), October 30, 2002


Early-formed ferromagnesian mineral crystals, like olivine and pyroxene, have rather high concentrations of Fe and Mg relative to SiO. Likewise, formation of Ca-rich plagioclase crystals at high temperatures removes Ca from the magma. Therefore, if they are removed from the magma, you're removing a lot of Fe and Mg relative to SiO, leaving the remaining material HIGHER in SiO. Minerals that contain greater proportions of SiO can form from the high-silica magma. For example, quartz is pure SiO2, and it forms in the latest stages of magma cooling of a rhyolitic magma.

Does that make sense?

-- Sharon Gabel (gabel@oswego.edu), October 31, 2002.

Yes i undertand since u explained the diagram in class today it seems to be much clearer now. Thank You.

-- Vanessa Azevedo (Nessi1024@aol.com), October 31, 2002.

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