The color of an igneous rock is related to the colors the minerals present. Rocks that are rich in silica usually contain considerable quartz and light-colored feldspar. Rocks rich in magnesium (Mg) and iron (Fe) tend to contain olivine, pyroxene, amphibole, and biotite. These minerals tend to be strongly (dark) colored.

On the basis of the amount of color versus white, a color index can be set up that relates to the composition. A rock that contains <30% colored minerals is considered light and may be called felsic (from feldspar and silica). With 30-60% colored minerals it is called mafic (from magnesium and ferro, meaning iron) and with >60%, ultramafic.

Steps in Identification Determine the color (indicates mineral composition) Determine the texture (indicates cooling history) Phaneritic=large grains Aphanitic=small grains (too small to identify with the naked eye) Porphyritic=fine grains mixed with larger grains Vesicular=holes Glassy=glass-like Example: A light-colored, fine-grained rock=rhyolite

Igneous Rocks

Rock Cycle