Description
Tourmaline (quartz) is a group of minerals that are all cyclosilicates. The minerals in this group share a characteristic chemical formula: The chemical formula is AX3Y6(BO3)3 Si6O18(O, OH, F)4. The A can contain either calcium or sodium. The X can be aluminium, iron, lithium, or magnesium. The Y is normally aluminium, but can also be chromium or iron. Some potassium can exist in the A position, manganese can be in the X, and vanadium can be found in the Y, but these elements are not commonly found in tourmaline group formulas. Tourmaline is found in pegmatites, metamorphites, magmatites, and alluvial deposits. Although tourmaline was known in the Mediterranean region since ancient times, it was not introduced into Western Europe by the Dutch from Sri Lanka until 1703. They named the new gemstone with a Sinhalese word “Turmali”, meaning “stone of mixed colours”. Rubellites were traditionally used as gemstones – they were used by artists as talismans, believed to enhance their creative powers. Tourmalines have certain unique properties. They are piezoelectric, meaning that when a crystal is heated or compressed, different electrical charges are formed at both ends of the crystal. This creates an electrical potential. When an external electrical potential is applied to the crystal, it vibrates. The minerals are pleochroic, meaning that the crystal appears darker in color when viewed along its longest axis than when viewed perpendicular to that axis. The four best-known and most common tourmaline minerals have varying colors and transparencies. Elbaite is transparent and a valuable gemstone. Schorl, rich in iron, is the most common mineral in the tourmaline group and is black and opaque. It is primarily formed in pegmatites, the extremely slowly cooling gangues of magma. The two other tourmalines commonly found are dravite and uvite. Dravite is typically translucent brown and can grow very large. Uvite is translucent green to opaque. Tourmaline is present in the sand fraction of Dutch Quaternary river sediments. It is a characteristic component of Meuse sands, among other things. In the heavy mineral analysis conducted in the Netherlands by the National Geological Survey during the second half of the twentieth century, the mineral is classified in the so-called stable group.
