The classification of igneous rocks is based not just on composition, but also on texture. As mentioned earlier, texture refers to the features that we see in the rock such as the mineral sizes or the presence of glass, fragmented material, or vesicles (holes) in the igneous rock. We will cover mineral crystal sizes and vesicles in this section.
Since the crystals or phenocrysts form while the magma is cooling, then the size of the crystals must have something to do with the cooling process. Recall that each mineral derives its chemical composition directly from the magma and that each mineral has a certain temperature interval during which that particular mineral can form. The chemical elements that become part of the mineral must migrate from the liquid magma to link or bond with other elements in a certain way to form the crystal structure that is unique for that mineral. What do you think will happen if the magma’s temperature drops quickly or if the magma’s temperature drops slowly? Either way, the time allowed for the migration of the chemical elements to form a crystal is affected. When magma cools slowly, there is plenty of time for the migration of the needed chemical elements to form a certain mineral; that particular mineral can become quite large in size, large enough for a person to see without the aid of a microscope. As a result, this igneous rock with its visible minerals is said to have a phaneritic texture (phan = large). The rock samples shown in Figure 8.2 are all phaneritic rocks. Figure 8.2A is a phaneritic mafic rock called gabbro, Figures 8.2B and 8.3B are a phaneritic intermediate rock called diorite, and the rock in Figure 8.2C is a phaneritic felsic rock known as granite. If you refer back to Figure 8.1 (Bowen’s reaction series), you will see that these rock names are listed on the right side of the diagram.
Magma that cools relatively quickly will have the opposite result as described above; there is less time for the migration of the chemical elements to form a mineral and, as a result, the minerals will not have time to form large crystals. Therefore, many small crystals of a particular mineral will form in the magma. Igneous rocks that are composed of crystals too small to see (unless you have a microscope) are called aphanitic igneous rocks. Figure 8.3C, 8.4A, and 8.4C are aphanitic rocks. Because Figure 8.3C and 8.4A are dark in color, they are mafic aphanitic rock called basalt. The felsic rock in Figure 8.4C is called rhyolite. It is important to note that basalt and gabbro are both mafic rocks and have the same composition but one rock represents a magma that cooled fast (basalt) while the other represents a mafic magma that cooled slowly (gabbro). The same can be said for the other rock compositions: the felsic rocks rhyolite and granite have identical compositions but one cooled fast (rhyolite) and the other cooled slower (granite). The intermediate rocks diorite (Figure 8.2B) and andesite (Figure 8.4B) also represent magmas that cooled slow or a bit faster, respectively. Sometimes, there are some visible crystals in an otherwise aphanitic rock such as the andesite in Figure 8.4B. The texture of such a rock is referred to as porphyritic, or more accurately porphyritic-aphanitic since it is a porphyritic andesite, and all andesites are aphanitic. Two different crystal sizes within an igneous rock indicate that the cooling rate of the magma increased. While the magma was cooling slowly, larger crystals can form. However, if the magma starts to cool faster, only small crystals can form. A phaneritic rock can also be referred to as a porphyritic-phaneritic rock if the phaneritic rock contains some very large crystals (ie. the size of your thumb!) in addition to the other visible crystals. In Figure 8.6 are two porphyritic rocks: a porphyritic-aphanitic basalt, and a porphyritic-phaneritic granite.
Sometimes the magma cools so quickly that there isn’t time to form any minerals as the chemical elements in the magma do not have time to migrate into any crystal structure. When this happens, the magma becomes a dense glass called obsidian (Figure 8.7A). By definition, glass is a chaotic arrangement of the chemical elements, and therefore not considered to be a mineral; igneous rocks composed primarily of glass are said to have a glassy texture. The identification of a glassy rock such as obsidian is easy once you recall the properties of glass; any thick glass pane or a glass bottle that is broken will have this smooth, curve-shaped pattern on the broken edge called conchoidal fracture (this was covered in your mineral chapter). Even though obsidian is naturally occurring, and not man-made, it still breaks in this conchoidal pattern. If you look closely at the obsidian in Figure 8.7A, you will see the curved (conchoidal) surfaces by noticing the shiny pattern on the rock. Obsidian appears quite dark in color regardless of its composition because it is a dense glass, and light cannot pass through this thick glass; however, if the edges of the obsidian sample are thin enough, you may be able to see through the glass.
In Figure 8.7B there is another igneous rock that is also composed primarily of glass due to a very fast rate of magma cooling. This rock is called pumice and is commonly referred to as the rock that floats on water due to its low density. The glass in this rock is stretched out into very fine fibers of glass which formed during the eruptive phase of a volcano. Because these fibers are so thin, they are easy to break (unlike the dense obsidian) and any conchoidal fractures on these fibers are too small to see without the aid of a microscope. Pumice can have any composition (felsic to mafic) but, unlike obsidian, the color of the pumice can be used to determine the magma composition, as felsic pumice is always light in color and mafic pumice will be dark in color. Mafic pumice with a dark grey, red or black color is also known as scoria.