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Sunday, May 15, 2011

The Different Types of Pegmatites

Pegmatite in mica schist.
Photo by Arlette

If we seem to be staying on the subject of pegmatites it is because they are very important in the production of gemstones. There are over 100 different types of gems that are produced from pegmatites. Most of these pegmatites are chemically the same as granite, and the term actually refers to their texture rather than their composition. There are three recognized varieties of granitic pegmatite, and one that is recognized in gabbro. All pegmatites containing crystals that are larger than 2 cm in diameter and can reach truly gigantic proportions. There is a feldspar mine in the Ural Mountains of Russia that has been producing feldspar from any single crystal for many years.

Pegmatites were classified in the mid-20th century by Buddington with a depth zone classification of granitic rocks in 1959. Cerny’s revision of 1991 is the most commonly when used today for the classification of pegmatites. Basically these classifications are based on the depth at which pegmatites were emplaced, metamorphic grade as well as minor element content. Under this classification Abyssal pegmatites are the highest grade that have been in place that either high or low pressure. Muscovite grade pegmatites were emplaced in high-pressure, but a lower temperature. Rare element pegmatites were emplaced at even lower pressures and temperature. In this classification the miarolitic pegmatites having large cavities were emplaced at shallow depths. The most gemstones are found in the miarolitic pegmatites within the enclosed cavities.

Beryl with microcline found in a pegmatite cavity.
Photo by Rob Lavinsky

The simple pegmatite is composed of feldspar, quartz and lesser amounts of muscovite mica. This is the simplest type of pegmatite that is found that can range from tiny veins a few millimeters wide to great masses of pegmatite that can cover several acres, and be meters wide. For the most part simple pegmatites have a very uniform composition, and little variance in the size of the crystals from one side to the other. Approximately 80% of all pegmatites found in nature are of this type. There are very few gemstones found in the simple pegmatites. Most of them contain white colored feldspar and milky quartz with sometimes bands of muscovite mica parallel to the sides of the vein. Under the above classification this type of pegmatite would fall under the classification of being an abyssal pegmatite.

Red tourmaline on lepidolite and quartz from a pegmatite cavity.
Photo by Rob Lavinsky

The rare element class of pegmatites is further divided into the sub classifications LCT and NYF. LCT pegmatites contain lithium, cesium and tantalum. NYf stands for a niobium, yttrium and fluorine. These are perhaps the rarest types of pegmatite. Most of the gemstones bearing pegmatites are of the LCT variety. The closer they form to the surface the more apt they are to have miarolitic cavities containing gemstones.

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