Carbonate Mineral Calcite

Associated Smithsonian Expert: Jeffrey E. Post, Ph.D.

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Photograph by Cara Santelli, Smithsonian Institution


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A mineral-containing conglomerate rock from Metamora, Michigan, USA

About Minerals in Sedimentary Rocks

The forces of weather on the surface of the Earth can change the minerals that make up rocks. When mechanical weathering processes destroy old rocks, softer silicate minerals such as olivine and plagioclase may dissolve away, leaving behind harder minerals, such as quartz. In chemical weathering, the atoms react with oxygen or water in the environment. (Think of iron exposed for several months to the water and air outdoors. The iron rusts, and rusting is a kind of chemical weathering.) In the presence of water, some minerals transform into more hydrous minerals. Other minerals, such as calcite (calcium carbonate), dissolve completely in water over time. Some silicate minerals, such as quartz and garnet, are more resistant to weathering than other silicates, such as feldspar and mica.

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Fluorite specimens, different colors, cut as gemstones

The Colors of Minerals

One of the most striking, yet least diagnostic, features of many minerals is their color. Well-formed mineral crystals span the entire rainbow of tinctures, from red (cinnabar, garnet) to yellow (sulfur), green (malachite), blue (azurite, lazurite), and violet (the amethyst variety of quartz). Minerals containing iron and magnesium are often dark brown or dark green. Impurities, trace amounts of elements that do not normally belong in the mineral, may change the overall color of a crystal. For instance, depending on the trace amounts of impurities it contains, quartz may look colorless (no impurities), light pink (titanium, iron, or manganese), milky white (tiny bubbles of gas or liquid), purple (iron), yellow (iron), or brown (extra silicon). However, multiple minerals may have almost the same color, so scientists must rely on other physical properties to make definite identifications of mineral specimens.