Igneous petrology is the study of igneous rocks, which are rocks that have formed through the solidification of molten magma. This field of geology is concerned with the composition, structure, and origin of igneous rocks, as well as the processes that form and alter them. Igneous petrology is important for understanding the history and evolution of the Earth’s crust, as well as the processes that take place within the Earth’s interior. It is also useful for identifying the sources of minerals and other resources that are found in igneous rocks.
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Chemical composition
There are several methods that can be used to determine the chemical composition of igneous rocks. One common method is X-ray fluorescence spectrometry, which involves bombarding the rock with X-rays and measuring the energy of the fluorescence emitted by the elements in the rock. This can provide information about the elemental composition of the rock, including the abundance of various metals and metalloids.
Another method is inductively coupled plasma mass spectrometry (ICP-MS), which involves vaporizing a sample of the rock and using a plasma torch to ionize the elements in the sample. The ions are then separated based on their mass-to-charge ratio and detected using a mass spectrometer, which allows for the precise measurement of the abundances of various elements in the rock.
Other methods that can be used to determine the chemical composition of igneous rocks include atomic absorption spectroscopy, X-ray diffraction, and neutron activation analysis.
Classification of Igneous Rocks
Igneous rocks can be classified based on several different criteria, including their chemical composition, mineralogy, and texture. One common method of classification is based on the relative abundances of silica (SiO2) and alkali metals (Na and K).
Rocks with high silica content and low alkali metal content are classified as felsic. These rocks tend to be light in color and are typically composed of minerals such as quartz, feldspar, and mica. Examples of felsic rocks include granite and rhyolite.
Rocks with low silica content and high alkali metal content are classified as mafic. These rocks tend to be dark in color and are typically composed of minerals such as pyroxene, olivine, and amphibole. Examples of mafic rocks include basalt and gabbro.
Rocks with intermediate silica and alkali metal content are classified as intermediate. These rocks are intermediate in color and are typically composed of a mix of felsic and mafic minerals. Examples of intermediate rocks include andesite and diorite.
Igneous rocks can also be classified based on their texture, which refers to the size, shape, and arrangement of the crystals in the rock. The three main types of texture are phaneritic, aphanitic, and glassy. Phaneritic texture refers to rocks with large, visible crystals, while aphanitic texture refers to rocks with small, microscopic crystals. Glassy texture refers to rocks that have a glassy appearance, with no visible crystals.
Extrusive and Intrusive Rocks
Igneous rocks can be classified as either extrusive or intrusive, depending on how they formed. Extrusive igneous rocks form when molten magma or lava cools and solidifies at or near the Earth’s surface. Because the magma cools quickly, the crystals that form are small and the rock has a fine-grained texture. Examples of extrusive igneous rocks include basalt and andesite.
Intrusive igneous rocks, on the other hand, form when magma cools and solidifies below the Earth’s surface. Because the magma cools slowly, the crystals that form are large and the rock has a coarse-grained texture. Examples of intrusive igneous rocks include granite and gabbro.
The difference between extrusive and intrusive rocks can also be seen in their mineralogy. Extrusive rocks tend to contain more mafic minerals, such as pyroxene and olivine, while intrusive rocks tend to contain more felsic minerals, such as quartz and feldspar
QAPF Diagram
The QAPF (Quartz, Alkali feldspar, Plagioclase, Feldspathoid) diagram is a classification system for igneous rocks based on the relative proportions of quartz, alkali feldspar, plagioclase feldspar, and feldspathoid minerals. It is commonly used to classify intrusive rocks, such as granites, diorites, and gabbros.
The QAPF diagram is divided into four fields, each representing a different class of rock based on the relative proportions of the four minerals. The fields are as follows:
- Q: quartz-rich rocks with more than 20% quartz
- A: alkali feldspar-rich rocks with more than 90% alkali feldspar
- P: plagioclase-rich rocks with more than 90% plagioclase feldspar
- F: feldspathoid-rich rocks with more than 10% feldspathoid minerals
The QAPF diagram is useful for identifying the main mineralogy of a rock and for estimating the conditions under which the rock formed. It is also useful for comparing the compositions of different rocks and for classifying them into broad categories based on their mineralogy.
Volcanic and Plutonic Rocks
Volcanic rocks are a type of extrusive igneous rock that form from molten magma or lava that has erupted and cooled at the Earth’s surface. These rocks are characterized by their fine-grained texture and their high content of mafic minerals, such as pyroxene and olivine. Examples of volcanic rocks include basalt, andesite, and rhyolite.
Plutonic rocks, on the other hand, are a type of intrusive igneous rock that forms from magma that cools and solidifies beneath the Earth’s surface. These rocks are characterized by their coarse-grained texture and their high content of felsic minerals, such as quartz and feldspar. Examples of plutonic rocks include granite, gabbro, and diorite.
The difference between volcanic and plutonic rocks is largely due to the difference in the rate at which they cool and solidify. Volcanic rocks cool and solidify quickly, while plutonic rocks cool and solidify more slowly. This difference in cooling rate results in the different textures and mineralogies of these two types of rocks.
Minerals in Igneous Rocks
Igneous rocks are composed of a variety of minerals, which are naturally occurring inorganic substances that have a specific chemical composition and a specific crystal structure. The minerals present in an igneous rock will depend on the chemical composition of the magma from which the rock formed and the conditions under which the magma cooled and solidified.
Some common minerals that are found in igneous rocks include:
- Quartz: a common mineral that is made of silicon and oxygen (SiO2). It is typically found in felsic rocks such as granite.
- Feldspar: a group of minerals that are made up of a combination of aluminum, silicon, oxygen, and various other elements. Feldspars are common in both felsic and intermediate rocks.
- Pyroxene: a group of minerals that are made up of a combination of silicon, oxygen, and various other elements. Pyroxenes are common in mafic rocks such as basalt.
- Olivine: a mineral that is made up of a combination of iron, magnesium, silicon, and oxygen. It is common in mafic rocks such as basalt.
- Amphibole: a group of minerals that are made up of a combination of silicon, oxygen, and various other elements. Amphiboles are common in mafic rocks such as gabbro.
- Mica: a group of minerals that are made up of a combination of aluminum, silicon, oxygen, and various other elements. Micas are common in felsic and intermediate rocks.
Primary and Accessory Minerals
In geology, primary minerals are the minerals that make up the majority of the volume of a rock and are responsible for the rock’s major properties and characteristics. These minerals typically formed during the initial crystallization of the magma from which the rock formed.
Accessory minerals, on the other hand, are minerals that are present in a rock in smaller amounts and are not responsible for the rock’s major properties and characteristics. These minerals may have formed during the crystallization of the magma, or they may have been introduced into the rock after it solidified through processes such as alteration or metamorphism.
In igneous rocks, the primary minerals are typically the minerals that formed during the initial crystallization of the magma. These minerals may include quartz, feldspar, pyroxene, olivine, and amphibole, among others. Accessory minerals in igneous rocks may include micas, garnets, and apatite, among others.
The relative proportions of primary and accessory minerals in a rock can provide information about the conditions under which the rock formed and the history of the rock. For example, a rock with a high proportion of accessory minerals may have formed from magma that cooled and solidified slowly, or it may have undergone significant alteration after solidification.
