7.4: Composition

Composition

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Composition of Magma

Remember that before an igneous rock can form, there must be molten material known as magma or lava. Most rocks (with very few exceptions) contain minerals that are crystalline solids composed of the chemical elements. From Chapter 3, recall that the most common minerals belong to a group known as the silicate minerals Links to an external site., which are abundant in the rocks of the lithosphere and mantle. Magma that forms from melt in these layers of the Earth also contain these same silicate minerals. All minerals, not just the silicates, are characterized by a certain set of conditions, such as temperature, at which they can melt. Recall that rocks are an aggregate or mixture of minerals. Oftentimes this can result in only some of the minerals in a rock melting, while others stay solid. Temperature conditions are important, as only minerals that melt at “lower” temperatures (590°C/1100°F) may experience melting, whereas the temperature would have to increase (for example, to 1150°C/2100°F) in order for other minerals to also melt. At these higher temperatures, even the lower temperature minerals are still melting and thus adding their chemical components to the magma generated. Even if the same types of rocks are melting, different magma compositions are generated by melting at different temperatures; we refer to this as magma evolution.

Magma is more buoyant than the source rock that generated it, and will eventually start to rise upward through the Earth’s lithosphere. This separation of the magma from the source region will result in new thermal conditions. The magma moves away from the heated portion of the lithosphere and encounters cooler rocks, which results in the magma also cooling. As with melting, minerals also have a certain set of conditions at which they form, or crystallize, from a cooling melt. You would be right in thinking that the sequence of mineral crystallization is the opposite sequence of crystal melting. The sequence of mineral formation from magma was experimentally determined by Norman L. Bowen in the early 1900’s, and the now famous Bowen’s Reaction Series Links to an external site. appears in countless textbooks and lab manuals (including this one!; Figure 7.5).

This “reaction series” refers to the chemical reactions that form the minerals, through chemical bonding of elements within the magma, in a sequence that is based on falling magma temperatures. Close examination shows that the first mineral to crystallize in a cooling magma of ultramafic composition is olivine Links to an external site.. Once temperatures fall below a specific range, olivine crystals will no longer form; instead, other minerals such as pyroxene Links to an external site. will start to crystallize (a small interval of temperatures exists where both olivine and pyroxene can crystallize). Minerals that form in cooling magma are called crystals. As these crystals are forming, they are removing chemical elements from the magma. For example, olivine crystals take magnesium (Mg) and iron (Fe) from the magma and incorporate them into their crystal structure. This behavior of mineral crystals to take certain chemical elements into their structure, while excluding other elements, means that the composition of the magma must be changing as crystals are forming. This is why magmas continuously evolve: their chemistry is always changing.

There can be more than one mineral type crystallizing within the cooling magma. The minerals on the left side of Bowen’s Reaction Series are referred to as a discontinuous series, as these minerals (olivine, pyroxene, amphibole, and biotite) all remove the iron (Fe), magnesium (Mg), and manganese (Mn) from the magma during crystallization, but do so at certain temperature ranges. These iron- and magnesium-rich minerals are referred to as the ferromagnesian silicates (ferro = iron) and are typically green, dark gray, or black in color due to the absorption of visible light by the iron and magnesium atoms. On the right side of Bowen’s reaction plagioclase crystallizes over a large temperature interval and represents a continuous series of crystallization even though its composition changes from calcium (Ca) rich to sodium (Na) rich. As the magma temperature drops and plagioclase first starts to crystallize (form), it will take in the calcium atoms into the crystal structure, but as magma temperatures continue to drop, plagioclase takes in sodium atoms preferentially. As a result, the higher temperature calcium-rich plagioclase is dark gray, in color due to the high calcium content (anorthite, or labradorite if also iridescent). The lower temperature sodium-rich plagioclase is white or lighter gray due to the high sodium content (albite).

 

Table 7.1: Plagioclase group minerals and their percentage of sodium (Na) and calcium (Ca). (CC-BY 4.0; Chloe Branciforte)

Mineral name	% Albite (Na-rich)	% Anorthite (Ca-rich)
Albite	100-90	0-10
Oligoclase	90-70	10-30
Andesine	70-50	30-50
Labradorite	50-30	50-70
Bytownite	30-10	70-90
Anorthite	10-0	90-100

 

Finally, at the bottom of Bowen’s Reaction Series, we see three more minerals form as temperatures continue to drop. These minerals (potassium feldspar, muscovite, and quartz) are considered to be “low temperature minerals”, as they are the last to form or crystallize during cooling; conversely these three minerals are the first to melt as a rock is heated. The removal of iron and magnesium from the magma (minerals of the discontinuous series) results in the formation of these last three minerals and magma that is deficient in these chemical elements. Oftentimes, these minerals are referred to as the nonferromagnesian silicates, because they lack iron and magnesium and are considerably lighter in color; for example, potassium-rich feldspar (also known as K-feldspar or K-spar) can be pink, white, or beige in color. 

 

Table 7.2: The potassium feldspar group minerals. (CC-BY 4.0; Chloe Branciforte)

Mineral name	Chemical formula
Sanidine	KAlSi3O8
Orthoclase	KAlSi3O8
Microcline	KAlSi3O8

 

The reference to mineral color is necessary, because the color of any mineral is primarily due to the chemical elements present in the mineral. Therefore, the color of an igneous rock will be dependent on the mineral content (or chemical composition) of the rock.

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Igneous Rock Composition

Bowen’s Reaction Series also includes the igneous rock compositions, which are entirely dependent on the minerals that compose that rock. For example, you can expect to find abundant olivine, and maybe a little pyroxene and a little Ca-rich plagioclase, in an ultramafic rock called peridotite or komatiite. Pyroxene, plagioclase, amphibole, and possibly some olivine may be present in a mafic rock such as gabbro or basalt. You would expect to see quartz, muscovite, potassium feldspar, and maybe a little biotite and Na-rich plagioclase in a felsic (or silicic) rock such as granite or rhyolite.

The classification of an igneous rock depends partly on the minerals that may be present in the rock. Since the minerals have certain colors due to their chemical makeup, then the rocks must also have certain colors. For example, a rock composed mostly of olivine will be green in color (like Mountain Dew) due to olivine’s green color; such a rock would be considered to have an ultramafic Links to an external site. composition. A rock that has a large amount of ferromagnesian minerals in it will be dark-colored, because the ferromagnesian minerals (besides olivine) tend to be dark colored. An igneous rock that is dark in color is referred to as having a mafic Links to an external site. composition (“ma-” comes from magnesium, and “fic” from ferric iron). An igneous rock with a large amount of nonferromagnesian minerals will be lighter in color; these are termed silicic or felsic Links to an external site. in composition (“fel” from feldspar, and “sic” from silica-rich quartz). A rock with a composition of intermediate Links to an external site., will fall between mafic and felsic. Using color, we’ll be able to properly identify any crystalline igneous rock (i.e. a rock contains crystals or minerals). Non-crystalline igneous rocks require a bit more information. As previously mentioned, classifying rocks into one of the igneous rock compositions (ultramafic, mafic, intermediate, and felsic) depends on the minerals that each rock contains.

An easy method of determining igneous rock composition is through determining the percentage of dark-colored minerals in the rock, without trying to identify the actual minerals present; this method of classification relies on a mafic color index Links to an external site. (MCI), where the term mafic refers to any dark gray, black or green colored mineral. Igneous rocks with 0-15% dark colored minerals (or 0-15% MCI) are felsic. Igneous rocks with 46-85% MCI are mafic. Igneous rocks with over 85% MCI are considered ultramafic. This means that any rock with between 16-45% MCI has an intermediate composition.

Estimating the percentage of dark-colored minerals is only possible if the minerals are large enough to see. In general, we can distinguish a mafic rock by its dark-colored appearance and a felsic rock by its light-colored appearance. An intermediate rock will be somewhat lighter than a mafic rock, yet darker than a felsic rock. Finally, an ultramafic rock is typically green in color, due to the large amount of olivine or pyroxene in the rock. Rocks that contain minerals that are too small to be seen can still be distinguished between ultramafic, mafic, intermediate and felsic by their overall color (Table 7.3). 

 

Table 7.3: The mafic color index. (CC-BY 4.0; Chloe Branciforte)

Composition	% of mafic minerals	Overall color	Aphanitic (extrusive)	Phaneritic (intrusive)
Felsic	0-15	Light - pink, white, beige	Rhyolite Trachyte	Granite Syenite Monzonite
Intermediate	16-45	Gray, green	Dacite Andesite	Granodiorite Diorite
Mafic	46-85	Dark - black, green, grey	Basalt	Gabbro
Ultramafic	>85	Green (olivine)	Komatiite	Peridotite