4.2.4: Metamorphic Minerals

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4.22.jpg — 4.22 A large red garnet in a mylonite

Metamorphism sometimes involves recrystallization and coarsening of a rock with no change in mineralogy. Often, however, it involves chemical reactions and replacement of preexisting minerals by new ones. Bonds are broken and atoms migrate by solid state diffusion or are transported short distances by intergranular fluids to sites where new minerals crystallize and grow. The photo shows a large red garnet crystal in a highly deformed metamorphic rock called a mylonite. Large minerals of this sort in metamorphic rocks are called phenocrysts.

The mineralogy of metamorphic rocks is more diverse than in sedimentary or igneous rocks. Nearly all the minerals found in igneous rocks can be present in metamorphic rocks. Many minerals that are found in sedimentary rocks may be present as well. In addition, other minerals, uncommon or nonexistent in igneous and sedimentary rocks, form through metamorphism. The table below lists only a few of the more common metamorphic minerals.

Common Metamorphic Minerals (besides those common in igneous and sedimentary rocks)
mineral	chemical formula
cordierite tremolite wollastonite andalusite kyanite sillimanite staurolite chloritoid garnet zoisite	(Mg,Fe)₂Al₄Si₅O₁₈ Ca₂Mg₅Si₈O₂₂(OH)₂ CaSiO₃ Al₂SiO₅ Al₂SiO₅ Al₂SiO₅ Fe₂Al₉Si₄O₂₃(OH) (Fe,Mg)A_l2SiO₅(OH)₂ (Ca,Fe,Mg)₃(Al,Fe)₂Si₃O₁₂ Ca₂Al₃Si₃O₁₂(OH)

4.23 Green diopside with white dolomite and calcite

Metamorphism may involve replacement of one mineral by another. For example, calcite may become aragonite or vice versa. Both minerals are CaCO₃, but their atomic structures differ. Mineralogical changes due to metamorphism, however, usually involve several different minerals reacting together. Dolomite (CaMg(CO₃)₂) and quartz (SiO₂) may react to form diopside (CaMgSi₂O₆) if a limestone containing quartz is metamorphosed at high temperature. The photo here shows green diopside surrounded by dolomite and calcite.