Pegmatite

Pegmatite is the name for the coarse-grained variety of igneous rock ( " coarse-grained facies of any type of igneous rock" ). The typical grain size is more than a centimeter, but also minerals with sizes of well over one meter are not uncommon.

Features

The vast majority of pegmatites are granitic or alkaligranitischer composition and consist mainly of the minerals quartz, feldspars ( orthoclase, microcline and sodium -rich plagioclase ) and biotite micas such as muscovite or lepidolite. Quantitatively, subordinate, however, to some extent economically very significant, are mafic and syenitic pegmatites. Pegmatites typically form gang- or lens-shaped body of rock with thicknesses in the order of a few tens of centimeters to a few tens of meters. The gears can be zoned internally, that is, certain minerals tend to occur at the edge, others in the core of the pegmatite on.

Formation

Melting, leading to Pegmatitbildung arise if, during the crystallization of a pluton incompatible elements such as lithium, rare earths, thorium or uranium, that is generally not appropriate in the crystal structure of common rock-forming minerals ingredients and at the same time volatile substances such as water, phosphorus, boron, or fluorine in the residual melt to accumulate. A similar process takes place at high-grade metamorphism in the partial melting of rocks.

By the presence of the volatile components, the melting point is significantly reduced on the one hand, so that the melt solidifies only at a temperature of about 450 ° C, on the other hand it is less viscous and the viscosity is thus significantly lowered. Both are important, so that the melt can move along crevasses in the rock from the point of origin, often many kilometers.

Classification

Due to their chemical composition, mineral contents and their formation environment ( metamorphic facies ) can be ( granitic ) divide pegmatites in different classes.

• Abyssal pegmatites: Emergence by anatexis in amphibolite - to granulite environment and rarely contain minerals of the elements U, Th, Zr, Nb, SEE. Numerous pegmatites of this class are found in the highly metamorphosed Precambrian shields areas of the world.
• Muscovite pegmatites: Emergence by anatexis under amphibolite facies conditions. They occasionally include Li, Be, Ti, Nb, Ta ​​, U, Th and SEE minerals and are a source of industrial muscovite production.
• Rare element pegmatites: Emergence by progressive fractionation of granitic magmas in upper greenschist - amphibolite facies Lower. Frequently minerals of Li, Rb, Cs, Be, Ga, SEE, Sn, Ti, U, Th, Hf, Nb and Ta rare element pegmatites are depending on mineralogy further divided into 1) rare-earth pegmatites ( allanite, monazite, Gadolinit ); 2) beryl - bearing pegmatites (beryl, columbite ); 3) complexes ( Li ) pegmatites ( spodumene, Petalite, Amblygonite, Lepidolite ); 4) albite -spodumene pegmatites ( spodumene ); 5) albite pegmatites.
• Miarolitische pegmatites forming pockets in hypabyssalen plutons and make a way, a low-pressure variant of the rare element pegmatites dar.

Use

From pegmatites industrially important minerals and trace elements such as boron, beryllium and lithium are obtained. Sometimes are also in semi-precious stones such as topaz, smoky quartz, various tourmalines or beryl. Because of their ductal occurrence, the unevenly distributed mineralization and mostly low volume play pegmatites in a global comparison as deposit only a minor role. Mineral collectors appreciate pegmatites due to their sometimes very rare and / or unusually large minerals.

Important examples are the two-kilometer long and 45 to 60 meters thick Bikita pegmatite in Zimbabwe, which is one of the largest deposits of lithium, cesium and beryllium in the world. In the Altai Mountains in Mongolia, there is a 20 to 70 kilometer wide swarm belt, in the line up to a length of approximately 450 kilometers numerous Pegmatitfelder and swarms. Here especially deposits of rare earth metals are mined. The pegmatite of Bancroft (Ontario) and the huge deposits of Rössing in Namibia are degraded especially because of their content of uranium.