By Mark Marikos
As you may already know, the theme for our 2024 Tucson Gem and Mineral Show® is “Pegmatites: Crystals Big and Beautiful”. But you may be wondering, “What are pegmatites, and why did we choose them as our Show Theme”.
Unless you have taken at least one geology class, you may not know what a pegmatite is, and why pegmatites produce some of the most awesome mineral specimens in the world. So, I thought it might be helpful to explain what pegmatites are, how they form, and why that process produces such large and spectacular mineral crystals.
It all starts deep in the earth, generally in a region where two crustal plates are smashing together in what is called a subduction zone. When one of the plates is comprised of relatively thin, dense oceanic crust that is converging with a thicker, less dense continental crust plate, the ocean crust “slab” plunges beneath the continental crust and its underlying mantle rock.
Figure 1: Diagram of typical subduction zone, showing oceanic crust and mantle lithosphere plunging beneath a continent Source: Subduction-en.svg from Wikimedia Commons by K. D. Schroeder, CC-BY-SA 4.0
The ocean crust basalts and gabbros have been extensively altered by contact with ocean water and circulating hydrothermal waters. It is rich with water-bearing minerals (clays, chlorites, serpentine, etc.), and carbonate minerals like calcite and aragonite. The basalts and gabbros are overlain by water saturated oceanic sediments rich in clays and carbonate minerals. As these subducted rocks sink deeper into the mantle, they are heated and compressed causing them to release water and carbon dioxide.
At those conditions water and carbon dioxide behave very differently than they do near the surface. They become “supercritical fluids” that have the density and dissolving power of a liquid, but flow as freely as a gas.
So, they move easily between the mineral grains of the oceanic crust, the mantle into which it is being thrust, and the continental rocks overlying the subduction zone. As they migrate upward, they leach elements from the rocks through which they pass, and when they reach the igneous, metamorphic, and sedimentary rocks of the crust, they also bring the heat they absorbed from the mantle beneath it. The hot fluid makes the crustal rocks melt more easily to form magma.
Figure 2: This is migmatite, a rock that is transitional between a metamorphic and igneous rock. The dark banded gneiss portions (“melanosomes”) are residual metamorphic rock that have begun to melt, releasing a granitic magma which collects in seams and cross-cutting fractures that form a collection network of light-colored “leucosomes” that allows the new magma to “drain” out of the rock. Because of its lower density it begins to rise.
This magma is generally of granite or granodiorite composition that eventually rises and crystallizes into minerals such as feldspar, quartz, micas, pyroxenes, and amphiboles. Under certain conditions, the magma may reach the surface and erupt as a volcano. But more often it “stalls out” before it reaches the surface and forms a large mass of igneous rock called a pluton.
It stalls when it has crystallized enough to make it too “stiff” to flow easily, and its density approaches the density of the surrounding rocks. But there is still a significant proportion of the original magma that has not crystallized. This residual liquid continues to move upward and is enriched in “incompatible” elements that do not easily “fit” into the silicate rock-forming minerals crystallizing in the pluton.
The fluid that accumulates at the top and margins of the pluton contains a higher percentage of water and other volatile compounds than the original magma and may carry large concentrations of normally rare elements. Because it is dominated by those volatile compounds, it may collect in relatively large water-filled voids above and to the sides of the pluton (see Figure 3).
Under the right conditions, it begins to crystallize inward from the walls of those “veins” and cavities, and because it contains a lower concentration of rock-forming silicates, fewer crystals “nucleate”, so the crystals have more room to grow than they would in the pluton. That is a pegmatite.
Figure 3: Fluid collecting near the top of a cooling granitic pluton fills and expands fractures and voids and eventually crystallizes pegmatite veins and pods in the top of the granite and the surrounding host-rock.
Modified after figure 2 in Breaks, F.W., Selway, J.B. and Tindle, A.G., 2003, Fertile peraluminous granites and related rare-element mineralization in pegmatites, Superior Province, northwest and northeast Ontario: Operation Treasure Hunt; Ontario Geological Survey, Open File Report 6099, 179p.
If you took a geology or earth science class, you probably learned that the slower a rock cools and crystallizes, the larger the grains grow. Let me burst that bubble. Much recent research indicates just the opposite for pegmatites. A normal pluton may take thousands to tens-of-thousands of years to completely crystallize. It is now thought that a pegmatite may completely crystallize in years or months.
The reason that pegmatite crystals grow so large is that there are far fewer crystals that nucleate (start to grow) than in the same volume of magma, because the concentration of dissolved mineral matter is much lower. And the crystals have room to grow large, with well-defined faces, because they are crystallizing into a water-rich fluid filling the cavity or vein. Extreme examples include the 30+ foot-long spodumene crystals found in the Etta mine near Keystone, South Dakota in the Black Hills (http://www.mineral-exploration.de/mepub/etta.html).
The pegmatite-forming fluid is also greatly enriched in rarer elements like beryllium, phosphorus, boron, cesium, lithium, tantalum, niobium, zirconium, etc., so it also crystallizes large crystals of beryl, apatite, tourmaline, spodumene, zircon, muscovite, lepidolite, etc. that are often highly colored by the various metals that also became concentrated in the fluid.
The vivid colors contrast nicely with the lighter colored white, grey, or tan feldspars and quartz that they grow upon. The rapid crystallization also makes the fluid composition change rapidly, so that minerals like tourmaline, beryl, and micas may have strikingly beautiful zoning.
The pegmatite-forming processes, therefore, may produce large, well defined, and colorful crystals of uncommon minerals growing on a light-colored matrix of quartz and feldspar (which may also form large well-defined crystals of multiple habits) – creating spectacular mineral specimens, or large, clear, colorful crystals suitable for faceting top-quality gemstones.
And now you know why we chose Pegmatites as our theme for the 2024 Tucson Gem and Mineral Show®! Be prepared to see some awesome specimens in February.
