Texture describes the way a rock looks. Grain size is the main texture you look for when classifying igneous rock. Grain size is the size of the individual mineral crystals in the rock.Coarse-grained (phaneritic) describes rocks with large crystals. Most intrusive rocks are considered coarse-grained because of their large crystals.
Fine-grained (aphanitic) describes rocks with crystals that are too small to see with the naked eye. Most extrusive rocks are considered fine-grained rocks.
Rocks that cool quickly may have trapped bubbles of gas called vesicles. The texture of these rocks is vesicular.
Porphyritic texture rocks have both fine-grained and coarse-grained crystals., You can examine the chemical composition of a rock just by looking at it. Minerals vary in color, and the color tells you what chemicals are in the rock. The most common chemicals are silica, iron, and magnesium.Light colored minerals (often called felsic) have more silica in them and are often white, gray, pink, or colorless.
Dark-colored minerals (often called mafic) have more iron and magnesium in them are usually black, brown, dark gray, or green.
 Igneous rock can be classified as felsic, intermediate, mafic, and ultramafic. These classifications apply to both extrusive and intrusive rocks.Felsic rocks are the lightest, and ultramafic rocks are the darkest.Felsic rocks have the highest silica content (65% +). Common examples of felsic rocks are rhyolite (extrusive rock) and granite (intrusive rock).
Intermediate rocks have about 55-65% silica content and are a little darker than felsic rocks. Common examples of intermediate rocks are andesite/dacite (extrusive rock) and diorite/granodiorite (intrusive rock).
Mafic rocks are low in silica (45-55%) and are dark in color. Common examples of mafic rocks are basalt (extrusive rock) and gabbro (intrusive rock).
Ultramafic rocks have the lowest silica content (less than 45%) and may have a green tint. Common examples of ultramafic rocks are peridotite (intrusive rock) and komatiite (extrusive rock).
Examine the texture of the rock.
Examine the chemical composition.
Know the four classifications of igneous rock.
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