Magma crystallization
Magma crystallization is the process by which molten rock, known as magma, cools and solidifies to form igneous rock. This process typically occurs either deep within the Earth’s crust or at the surface when magma erupts as lava. As magma cools, minerals crystallize in a specific sequence based on temperature, beginning with those rich in calcium, iron, and magnesium, and progressing to those with higher concentrations of aluminum, potassium, and sodium. Slow cooling beneath the surface allows for larger crystal growth, resulting in rocks with a phaneritic texture, while rapid cooling at the surface produces fine-grained or glassy rocks, characterized as aphanitic.
Crystallization not only leads to the formation of rocks but also influences the concentration of valuable elements. For instance, certain minerals, like diamonds, originate from deep within the mantle and are transported to the surface by specific types of magma called kimberlite. Additionally, magma can form layered intrusions where minerals are organized in a repeating sequence due to gravity layering, contributing to major deposits of metals such as platinum and chromium. Overall, magma crystallization is a fundamental geological process that plays a crucial role in the formation of the Earth’s crust and the availability of various mineral resources.
Magma crystallization
Magma crystallization is a geologic process in which molten magma in the Earth’s interior cools and subsequently crystallizes to form an igneous rock. The crystallization process produces many different types of minerals, some of which are valuable natural resources.
Background
Magma is molten material consisting of liquid, gas, and early-formed crystals. It is hot (900° to 1,200° Celsius), mobile, and capable of penetrating into or through the Earth’s crust from the mantle, deep in the Earth’s interior. Most magma cools in the Earth’s crust; in a process similar to ice crystallizing from water as the temperature drops below the freezing point, minerals crystallize from molten magma to form a type of rock called igneous rock. Once completely crystallized, the body of igneous rock is called an intrusion. Some magma, however, works its way to the surface and is extruded as from volcanoes.
Mineral Growth
Magma that remains below the surface cools at a slow rate. Ions have time to collect and organize themselves into orderly, crystalline structures to form minerals. These minerals grow larger with time and, if the cooling rate is slow enough, may grow to several centimeters in diameter or larger. Igneous rocks with minerals of this size are said to have a phaneritic texture. Magma that reaches the surface, on the other hand, cools very rapidly and forms rocks that consist of extremely fine-grained minerals or quenched glass. These rocks have an aphanitic or glassy texture. Consequently, it is those minerals that grow beneath the surface that reach sizes large enough to be considered economically feasible resources.
Concentration of Valuable Elements
Minerals do not crystallize from magma all at once. Instead, they follow a sequence of crystallization as the temperature decreases. In general, silicate minerals (substances with silicon-oxygen compounds) with high contents of calcium, iron, and magnesium crystallize early, followed by minerals with high contents of aluminum, potassium, and sodium. Excess silica crystallizes last as the quartz. Bonding factors such as ionic size and charge prevent some elements from incorporation into early crystallizing minerals. Thus, they are more highly concentrated in the residual magma and become incorporated into the last minerals to crystallize, forming rocks called granites and pegmatites. These rocks may contain minerals such as beryl, spodumene, lepidolite, and uraninite, which include important elements such as beryllium, lithium, and uranium. Granites and pegmatites are also important sources for feldspar and sheet mica.
Diamonds and Kimberlites
Perhaps the best-known magmatic minerals are diamonds. Formed deep in the at extremely high temperatures and pressures, diamonds are carried by a certain type of magma as it violently intrudes upward through the crust, sometimes reaching the surface. Upon cooling and crystallizing, this magma forms a pipe-shaped igneous rock known as kimberlite. It is in kimberlites that most diamonds are found. Most kimberlite pipes are less than one square kilometer in horizontal area, and they are often grouped in clusters. Most of the known diamond-bearing kimberlite pipes are found in southern Africa, western Australia, Siberia, and Canada.
Magmatic Sulfide Deposits
Most major metals used in industry (copper, iron, lead, nickel, zinc, and platinum) are found in sulfide minerals, which are substances that contain metal-sulfur compounds. When magma is in the early stages of cooling and crystallizing underground, certain processes can cause droplets of liquid sulfide to form within it. These sulfide droplets attract metallic cations and concentrate them by factors ranging from 100 to 100,000 over their normal levels in the host magma. The droplets eventually cool and solidify to form sulfide minerals such as pyrite (“fool’s gold”), galena (lead sulfide), and sphalerite (zinc sulfide). Sulfide minerals such as these become important targets for mining because of their high concentration of metals.
Layered Magmatic Intrusions
Some magmas give rise to layered intrusions in which a specific sequence of minerals is repeated many times from bottom to top in a process called gravity layering (also called rhythmic layering). Dark-colored, heavier minerals such as pyroxene, olivine, and chromite concentrate near the base of each layer, grading to predominantly light-colored minerals such as plagioclase at the top. Each mineral sequence is a separate layer, averaging several meters thick and ranging from less than 2 centimeters to more than 30 meters. It has been suggested that the origin of gravity layering involves multiple injections of fresh magma into a crystallizing magma chamber, effectively replenishing the magma and allowing the same minerals to crystallize repeatedly.
The Bushveld intrusion in South Africa, one of the largest layered intrusions, contains multiple gravity layers and is more than 7,000 meters in total thickness. Layered intrusions contain the Earth’s main for chromium and platinum. In the Bushveld intrusion, chromium occurs in the mineral chromite, and platinum in platinum-iron alloys, braggite, and other platinum-metal compounds. The main source for platinum minerals in the Bushveld intrusion, and the source for approximately half the Earth’s supply of platinum, is the Merensky Reef, a layer of chromite and platinum minerals 1 meter thick and more than 200 kilometers long. Also present in the Bushveld intrusion is the mineral magnetite, which yields important elements used in steel manufacturing, such as iron and vanadium. The Bushveld intrusion accounts for almost 50 percent of the world’s production of vanadium.
Bibliography
Best, Myron G. Igneous and Metamorphic Petrology. 2d ed. Malden, Mass.: Blackwell, 2003.
Brown, Michael, and Tracy Rushmer, eds. Evolution and Differentiation of the Continental Crust. New York: Cambridge University Press, 2006.
Evans, Anthony M. Ore Geology and Industrial Minerals: An Introduction. 3d ed. Boston: Blackwell Scientific Publications, 1993.
Jahns, Richard H. and Albert M. Kudo. "Igneous Rock." Britannica, 12 Nov. 2024, www.britannica.com/science/igneous-rock/Assimilation. Accessed 23 Dec. 2024.
Naldrett, Anthony J. Magmatics Sulfide Deposits: Geology, Geochemistry, and Exploration. Berlin: Springer, 2004.
Pietranik, Anna, et al. "Zircon Reveals Diverse Trends of Magma Crystallization from Two Types of Early Post-Collisional Diorites (Variscan Orogen, NE Bohemian Massif)." Journal of Petrology, vol. 63, no. 7, July 2022, doi.org/10.1093/petrology/egac059. Accessed 23 Dec. 2024.
Schmincke, Hans-Ulrich. “Magma.” In Volcanism. New York: Springer, 2004.
Young, Davis A. Mind over Magma: The Story of Igneous Petrology. Princeton, N.J.: Princeton University Press, 2003.