The coarse-grained or phaneritic texture is an igneous rock texture in which you can see and distinguish individual minerals without a microscope, i.e., you can see these minerals with your naked eye.
Remember, texture in geology and Earth sciences describes how mineral crystals or grains are arranged and their shape or size. It does not describe the feel or quality of a surface, such as rough, smooth, slimy, gritty, etc.
Phaneritic is an adjective from the word phaner, which means visible. It implies crystals or grains are visible. Such must be large enough.
A phaneritic or coarse-grained texture indicates rocks mostly formed deep inside the crust where cooling was slow, and crystals grew faster than nucleation.
Nucleation is an initial process during which ions, atoms, or molecules arrange themselves into a characteristic minute or embryonic crystalline solid. Larger crystals can grow from this minute crystalline solid as more particles (atoms, ions, or molecules) are added.
Also, from the texture, you can deduce which crystals formed first and relative depths. Well-formed crystals with easy-to-notice faces, i.e., euhedral, formed first. They had the space to grow unhindered.
On the other hand, crystal sizes will increase with depth as the slowing rate decreases. However, other factors can influence these sizes, too.
What is phaneritic texture?
Phaneritic texture describes igneous rocks, with a vast majority of crystals large enough to see and distinguish with your naked eye.
You don’t need a petrological microscope to see or identify the minerals. However, for some, you may need a simple hand lens.
Also, most of these rocks have interlocking nearly the same grain size intergrowth and holocrystalline, i.e., made of only crystals with no glass.
However, they can only have minor amounts of minute minerals visible with a microscope, such as zircon or apatite. Also, some oxides, especially Fe-Ti oxides, can be smaller.
The typical mineral grain size in phaneritic texture is 0.1mm to 3 cm (0.004-1.18 inches). Some sources put it at 1/16mm to 3cm or 5cm.
Furthermore, you can further subdivide phaneritic texture according to grain sizes as follows:
- Fine-grained: Crystal diameter between 0.1-1mm. Such crystals are smaller than sugar granules.
- Medium grain: Crystal sizes range from 1mm to 5mm, about the size of sugar granules, to the thickness of peas. These textures are common in subvolcanic rocks, i.e., those that form near the surface at a depth of about 2km (1.2 mi), like diabase or microgabbro.
- Coarse-grained: Such rocks have a crystal diameter of about 5 to 30 mm (0.5 to 3 cm).
- Very coarse-grained: Very coarse-grained rocks have a crystal diameter of over 30mm (3cm). These rocks are known as pegmatites when crystals are unusually large.
Lastly, phaneritic rocks can have mineral crystals with different colors. Remember, various minerals, like feldspars, amphiboles, mica, pyroxenes, quartz, etc., have different colors.
Phaneritic rocks can have other textures.
Rocks with a phaneritic or coarse-grained texture can have other textures, too. Such textures include equigranular and inequigranular.
1. Equigranular phaneritic texture
Equigranular phaneritic will have a coarse-grained texture with nearly equal-sized anhedral to subhedral crystals. Granite aplite has this texture, and it is known as aplitic.
2. Inequigranular phaneritic
Inequigranular phaneritic rocks will have two conspicuously variable sizes, i.e., bimodal grain sizes. These sizes may be porphyritic or seriate.
a). Porphyritic phaneritic
It has two sets of crystal sizes, i.e., unusually large crystals called phenocrysts that are set in a finer but still coarse-grained groundmass or matrix.
A good example is some granodiorites. They will have large potassium-rich alkali feldspar crystals as large as 6 inches long set in a finer but coarse-grained or phaneritic quartz, biotite, and feldspar matrix.
However, this texture is more common in aphanitic rocks, i.e., those whose crystals are too small to see or distinguish with the naked eye.
Note: Some phenocrysts rich porphyritic glassy or aphanitic rocks with nearly euhedral phenocrysts can somewhat deceptively resemble phaneritic. However, a careful examination with a microscope or hand lens should reveal they are not.
c). Seriate texture
Some phaneritic rocks will show a continuous increase in size. For instance, some granites may have crystal grains that increase from microscopic zircon and apatites to 1mm Fe-Ti oxides to 2-3 cm or larger quartz, plagioclase, or alkali feldspar grains.
It happens when nucleation rates vary in minerals precipitate together. The larger, the slower the nucleation.
How do phaneritic textures form?
Phaneritic texture mostly forms from the slow cooling of magma deep beneath the Earth’s surface in large magma chambers forming plutons. The slow cooling rate gives time for large crystals to grow.
However, it can form in some extremely thick lava flows. For instance, gabbro, a coarse-grained textured rock, can form within very thick basalt lava flows. The thick lavas offer insulation. This allows interior parts to cool slowly, allowing large mineral crystals to grow.
Going beyond the obvious, you should know that crystallization involves two stages requiring supersaturation or supercooling.
The first stage is nucleation. It forms minute or tiny embryonic crystals upon which crystals can grow. This step determines the rate and requires more energy or more supercooling.
The second stage is crystal growth. It involves the addition of more atoms, atoms, or ions in an orderly manner. This stage requires less energy or less supercooling.
Therefore, a slow cooling rate will result in less supercooling. This will favor crystal growth over nucleation. Consequently, you will have fewer but larger crystals.
More supercooling would favor nucleation over crystal growth if the cooling rate were faster. This will create many tiny crystals or an aphanitic texture.
Any even faster quenching will impede both nucleation and crystal growth. Instead, the melt will freeze, creating an amorphous solid or glass.
Examples of phaneritic rocks
Rocks with coarse-grained or phaneritic textures are mostly plutonic. Examples include granite, gabbro, diorite, peridotite, granodiorite, pyroxenite, anorthite, syenite and nepheline syenite. Others are latite, norite, monzonite, tonalite, shonkinite, and trondhjemite.
As you have noticed, these rocks range from ultramafic to mafic and intermediate to felsic. Therefore, this texture isn’t limited to rocks of a particular composition or magma type.
References
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell