Composition

Composition of Igneous Rock

An observational evaluation of the composition of an igneous rock sample is most relevant to the crystalline textured igneous rocks. A scale of composition is typically described by the changing fractions of felsic and mafic mineral components.

A small number of minerals or mineral groups constitute a majority of the common igneous rocks with a crystalline texture.

Felsic Igneous Rocks

Felsic igneous rocks are composed of a small group of silicate minerals dominated by quartz, plagioclase group feldspars, potassium (K) feldspars, and muscovite group mica. These minerals do vary in color but because they largely lack large amounts of iron (Fe) and magnesium (Mg), igneous rocks dominated with these minerals typically are lighter in tone (not color).

Mafic Igneous Rocks

Mafic igneous rocks are composed of another small group of silicate minerals dominated by pyroxene, amphibole group, biotite group and olivine group minerals. These minerals also vary in color but because they contain larger amounts of iron (Fe) and/or magnesium (Mg), igneous rocks dominated with these minerals typically are darker in tone (not color).

Rock Color Converts to Tone

Rock Color

Igneous rocks may be composed of a few or many different minerals that vary in color. The overall color of an igneous rock is controlled by the colors and abundances of the rock's component minerals.

However, it is important to not impose the actual colors of individual felsic and mafic minerals on the scale of mineral composition. Rather, the composition scale varies in tone - the overall lighter or darker appearance of the rock.

Igneous Rock Color Variation

Tone = Felsic

Tone = Mafic

Mafic composition

The composition of mafic igneous rocks is dominated by ferromagnesian (rich in iron and magnesium) minerals, (e.g., hornblende, pyroxene, olivine, among others). These minerals vary in composition and bulk color (e.g., dark brown, dark green, black). Most appear darker toned. Variations in actual color, like these, require us to focus on the tone, not the color, of the rock to make a judgment about composition.

Phaneritic Gabbro and Aphanitic Basalt

• Gabbro and basalt are both mafic in composition - both are darker-toned.
• Both are crystalline.
• Gabbro and basalt are distinguished by crystal size.
  • gabbro is phaneritic (crystals large enough to be seen without magnification)
  • basalt is aphanitic (microscopic crystals not visible to the naked eye)

In the simplified classification, the mafic (darker toned) crystalline igneous rocks are gabbro (phaneritic) and basalt (aphanitic). Watch this short video to learn how to distinguish gabbro and basalt.

Intermediate composition

The composition of the intermediate crystalline igneous rocks is, as the name suggests, a mixture of felsic and mafic mineral components. A nearly continuous gradient of mineral composition is possible, making the recognition of boundaries between felsic and intermediate, and intermediate and mafic, one of an arbitrary distinction.

Geologists have tackled this problem of continuous change by specifying boundaries between these categories based on specific numerical percentages of felsic and mafic mineral components. The percentage of a particular mineral component can be determined with quantitative methods such as point-counting.

In this identification activity, however, we will keep the procedure for classifying an igneous rock as felsic, intermediate or mafic by a qualitative eyeball estimate, largely by training on a small suite of images that serve to represent each discrete category. Errors in identification are more likely towards the boundaries between the categories than at the ends.

Variation in Tone

We use tone for the degree to which a crystalline igneous rock varies between light and dark (not of a specific color). Crystalline rocks that are a mixture of lighter and darker minerals large enough to be seen individually may be thought of like a mixture of salt (white) and pepper (black). If a rock is dominated by 'salt' it would appear overall lighter. If dominated by pepper, the mixture would appear overall darker.

We will not be concerned with a quantitative estimate of tone from a mixture of lighter and darker minerals. Instead, we'll try to calibrate our evaluation to just three categories - lighter = felsic, intermediate, and mafic=darker.

Here is a diagram to help calibrate our visual evaluation.

For larger view, click image to open in new tab.

Phaneritic Granite, Diorite and Gabbro

• Granite, diorite and gabbro are phaneritic rocks because they solidify slowly during which time crystals grow large enough in size to be visible to the naked eye.
• These rocks form a series that varies in composition, from felsic to mafic because of an increasing fraction of iron+magnesium rich minerals that darkens the tone.

In the simplified classification, the intermediate (intermediate toned) phaneritic, crystalline igneous rock is diorite. Watch this short video to learn how to distinguish diorite from granite and gabbro.

Aphanitic Rhyolite, Andesite and Basalt

• Rhyolite, andesite, and basalt are aphanitic rocks because they solidify quickly. This quenches the growth of crystals at the microscopic size. These rocks are called aphanitic because they are fine-grained.
• Rhyolite, andesite, and basalt form a series that varies in composition, from felsic to mafic because of an increasing fraction of iron+magnesium rich minerals that darkens the tone.

In the simplified classification, the intermediate (intermediate toned) aphanitic, crystalline igneous rock is andesite. Watch this short video to learn how to distinguish andesite from rhyolite and basalt.