Pegmatite: Composition, Formation and Occurrence

Have you ever wondered how precious gems such as Emerald, Tourmaline, Aquamarine, and topaz are formed or where exactly are they found, then this is a blog for you as I will be discussing the formation and the source of these precious gems?

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Emeralds in a rock


What is Pegmatite?

This is a type of igneous rock that is formed during the late magmatic processes, it is similar to granite except that, it has larger crystals greater than 3cm or more of variable sizes. What makes this rock interesting is its mineral and elemental content, popularly known for its gemstones minerals such as Topaz, Beryl, Amblygonite, Tourmaline, and industrial minerals such as quartz and feldspars. Some pegmatites are known to have incompatible elements including the rare earth elements but in this blog, our main discussion will be on gemstone mineral pegmatite with a focus on the complex or zoned pegmatites, reason been it consists mostly of the essential elements that act as ingredients for the formation gemstone mineral pegmatites.

The elemental composition of gemstone-based pegmatite(Zoned Pegmatite).

The elemental composition of these pegmatite relating to the complex or zoned pegmatite includes Chlorine, phosphorous, fluorine, Bromine, large ion lithophile, and rare earth elements with incompatible elements that are fractionated during magmatic fractional crystallization. The composition of the incompatible elements that make up colored gemstone minerals is, Sn, Cs, W, Rb, and Ta while the bulk composition of the rock, is similar to that of granite rock.

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Main group elements of the periodic table

Formation of Pegmatites. 

During the last stages of magmatic differentiation as the magma is cooling, there is an increase in water content in the granite magma, these fluids have a high concentration of incompatible elements, by incompatible elements I mean elements that failed to fit or were rejected to fit in the crystal lattice due to their large ionic radius or high valency charge, during the early stages of magmatic differentiation at high temperature and pressure, hence were concentrated within the fluid.

 These elements start to migrate by diffusion and start to bond and nucleate to form crystals, over geological time larger crystals are formed, the crystals that are formed are then enveloped in open cavities or pods which are solidified and differentiated lens-shaped bodies of water-saturated. Pegmatites are formed at shallow depth tend to have gemstone, optical fluorite, and quartz-rocks others are formed within patches of their parent granite or through thick dykes that are many kilometers long, others form in fractured-filling bodies of the country-rock and also can be found within the amphibolite metamorphic facies.

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Fractional Crystallization

Occurrence of Pegmatite.

The popular location known to have complex types of pegmatites includes the Zimbabwe Bikita pegmatite in the Zimbabwe craton and the Fort victoria Greenstone belt.

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