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Tourmaline WikiWhat is tourmaline?Tourmaline is a crystal silicate mineral compounded with elements such as Sodium, Iron, Aluminum, magnesium, Calcium, Lithium, or Potassium. All tourmaline varieties has the same crystal structure but differing in chemical composition, color, and other physical and optical properties. Tourmaline Crystal SystemMembers of the tourmaline group have the same basic crystal structure. Tourmaline belongs to the trigonal crystal system and is usually found as prismatic crystals with well developed trigonal prism and second-order hexagonal prism faces. Tourmaline crystals can be elongated or flattened, commonly vertically striated. The style of termination at the ends of crystals is asymmetrical. In cross section Prisms faces often have heavy vertical striations that produce a rounded triangular effect. Doubly terminated tourmaline crystals often show different forms at each end of the vertical axis. Tourmaline VarietiesThere are currently 14 tourmaline end members that have been accepted by the International Mineralogical Association (IMA) Inclusions in TourmalineInside a Tourmaline crystal are a number of different mineral inclusions that vary in shape as well as content. There are elongated or threadlike cavities, Mica, Apatite or Zircon crystals and sometimes hornblende inclusions. Some specimens have two phase inclusions consisting of a liquid and a gas bubble. Inclusions run parallel to the length of the crystals and when densely packed may produce a chatoyant effect that yields cats- eye gems when fashioned as cabochons. Tourmaline GeologyTourmaline is found in two main geological occurrences. Igneous rocks, in particular granite and granite pegmatite and in metamorphic rocks such as schist and marble. Tourmaline pegmatite is formed as a course grained granite by slow cooling of magma at a considerable depth in the Earth’s crust. These rocks whose slower rate of cooling in the middle and lower parts of the crust, made it possible for quite large crystals to form from the original molten residues. As the temperature of the magma began to drop, minerals separated out in a process known a fractional crystallization. Tourmaline was not one of the first to solidify but as the magma continued to cool, the rock contained the space necessary for Tourmaline crystals to form. Gem quality tourmaline crystals usually form in pocket zones or veins. If no pockets were present, the mineral is usually cracked and included, most often not suitable for gemstones. Tourmaline occurs in pegmatites with other minerals such as Lepidolite, Microline, beryl pollucite, albite and Spodumene. It has been found with Andalusite, Biotite, Siderite, Quartz, Molybdenite, manganotantalite, columbite-tantalite, and Cassiterite in other types of mineral formations. Tourmaline deposits can also be found as alluvial deposits after the minerals have been washed down from their original sights by rain, wind and sometimes fast moving rivers. |
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