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The term pegmatite is thus purely a textural description. [6] [7] Geologists typically prefix the term with a compositional description, so that granitic pegmatite is a pegmatite with the composition of granite while nepheline syenite pegmatite is a pegmatite with the composition of nepheline syenite. [6]
The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Examples of aphanitic igneous rock include basalt , andesite , and rhyolite . Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur.
Orthoclase crystal twinning from the Organ Mountains in New Mexico. Orthoclase is a common constituent of most granites and other felsic igneous rocks and often forms huge crystals and masses in pegmatite. Typically, the pure potassium endmember of orthoclase forms a solid solution with albite, the sodium endmember (NaAlSi 3 O 8) of plagioclase.
The way a mineral splits (or “cleaves”), particularly along planes in the crystal structure. Cleavage is generally described by. how well a mineral can be split to produce a flat plane, a process controlled by planes of weakness in the crystal structure. the number of distinct directions of these cleavage planes; the angles between those ...
Lepidolite is associated with other lithium-bearing minerals like spodumene in pegmatite bodies. It is the major source of the alkali metal rubidium . [ 13 ] In 1861, Robert Bunsen and Gustav Kirchhoff extracted 150 kg (330 lb) of lepidolite to yield a few grams of rubidium salts for analysis, and therefore discovered the new element rubidium.
MVT deposits are believed to form in very shallow water, most likely less than 50 meters in depth, whereas sedex deposits can form under relatively deep marine conditions; Mineralization is characterized by coarse grain size, cavities, breccia fragments and euhedral crystals. In contrast, sedex mineralization is commonly fine grained and laminated
Large crystals of mica used for various applications are typically mined from granitic pegmatites. [ 6 ] The largest documented single crystal of mica ( phlogopite ) was found in Lacey Mine, Ontario , Canada ; it measured 10 m × 4.3 m × 4.3 m (33 ft × 14 ft × 14 ft) and weighed about 330 tonnes (320 long tons; 360 short tons). [ 18 ]
The central portions of pegmatites are often miarolitic as the pegmatite dike crystallizes from the outside walls toward the center. The volatile portion of the magma is gradually excluded from the forming crystal phases until it becomes trapped within the body and forms the cavities which often contain minerals of elements incompatible with ...