The spinel species magnetite (Fe3O4) is found in a huge range of different rock types. The equilibrium between quartz, olivine, and magnetite is used as a reference to specify a commonly-occuring oxygen pressure typical of rocks formed near the Earth's surface. In addition, magnetites in many igneous and metamorphic rocks are titaniferous (containing ulvospinel, Fe2TiO4, in solid solution). Titaniferous magnetites coexisting with ilmenite solid solution are sensitive to both temperature and redox conditions (oxygen fugacity), and in favorable conditions the compositions of both phases can indicate the temperature at which they formed.
from: Mineralogy and Optical Mineralogy (2008) Dyar, M. and Gunther, M.; Mineralogical Society of America.
Showing posts with label spinel. Show all posts
Showing posts with label spinel. Show all posts
19.1.10
Spinel
Spinel (MgAl2O4) At about 400 km, the olivine structure is no longer stable. It changes spontaneously from the olivine structure to a spinel-type structure, which takes up 8% less space; the composition remains the same (though we now call it beta-Mg2SiO4). This causes seismic discontinuity at about 300-400 km depth that is the boundary tranistion zone.
from: Mineralogy and Optical Mineralogy (2008) Dyar, M. and Gunther, M.; Mineralogical Society of America.
from: Mineralogy and Optical Mineralogy (2008) Dyar, M. and Gunther, M.; Mineralogical Society of America.
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