Tourmaline is a highly complex borosilicate mineral that meets the strict geological criteria defining a mineral. It is widely recognized for its exceptional diversity in color, making it one of the most popular gemstones globally. The name is derived from the Sinhalese word turmali, meaning “mixed colored stones,” which speaks directly to its varied appearance. Its unique crystalline structure and chemical makeup place it within the cyclosilicate group of minerals.
Defining Characteristics of a True Mineral
The classification of any substance as a mineral is governed by five fundamental scientific criteria. First, a mineral must be naturally occurring, formed by geological processes without human intervention. Second, it must be inorganic, excluding materials derived from living organisms, such as coal or amber.
Third, a mineral must exist as a solid under normal surface conditions, though ice qualifies as a mineral. The fourth criterion is an ordered internal structure, where atoms are arranged in a precise, three-dimensional, repeating pattern known as a crystal lattice. This crystalline structure distinguishes minerals from amorphous solids like glass.
The final requirement is a defined chemical composition, which can be expressed by a chemical formula. While the composition must be predictable, it does not have to be fixed; many minerals allow for a variable substitution of elements within a specific range.
Tourmaline’s Complex Chemical Identity
Tourmaline’s chemical nature is one of the most complex in mineralogy. It belongs to the cyclosilicate group, characterized by six silicon-oxygen tetrahedra rings (Si6O18) arranged in a specific pattern. The general formula for the tourmaline group is extensive, incorporating elements like aluminum, boron, silicon, and various metals.
This complexity stems from its accommodating crystal structure, which allows for extensive chemical substitution, a phenomenon known as a solid solution series. Different species are defined by which elements occupy specific atomic sites, such as sodium, calcium, iron, magnesium, or lithium. For example, the black variety, schorl, is rich in iron, while the pink and green gem variety, elbaite, is rich in lithium and aluminum.
The internal atomic arrangement crystallizes in the trigonal system, often forming elongated prismatic crystals. A defining feature is the presence of heavy vertical striations running parallel to the long axis. This unique structure allows the mineral to incorporate a wide array of trace elements, accounting for its tremendous range of colors.
Observable Physical and Electrical Traits
Tourmaline displays distinctive physical properties linked to its atomic structure and composition. On the Mohs scale, it ranks between 7.0 and 7.5, making it durable for jewelry. Its specific gravity typically falls between 3.06 and 3.32, varying based on heavy metal content; iron-rich varieties are denser than lithium-rich ones.
The mineral is allochromatic, meaning its color is caused by trace impurities rather than being an inherent part of its chemical formula. Many crystals also exhibit strong pleochroism, an optical property where the color appears to change when the crystal is viewed from different angles. For example, a green tourmaline may look much darker or brownish when viewed down its vertical axis.
Tourmaline also possesses remarkable electrical properties, specifically pyroelectricity and piezoelectricity, stemming from the lack of a center of symmetry in its crystal structure. Pyroelectricity is the ability to generate a temporary electrical charge when heated or cooled. Piezoelectricity is the capacity to generate an electrical charge when subjected to mechanical stress or pressure.
Formation, Location, and Uses
Tourmaline typically forms under conditions involving high heat and the presence of boron-rich fluids. The most significant occurrences are found in granite pegmatites, which are coarse-grained igneous rocks formed during the final stages of magmatic crystallization. It is also found as an accessory mineral in metamorphic rocks, such as schist and marble.
Major sources of gem-quality tourmaline include Brazil, Afghanistan, Pakistan, and the United States (California and Maine). Its durability allows it to resist weathering, and it is sometimes found as resistant grains in sedimentary deposits.
The primary use for tourmaline is as a gemstone for jewelry, owing to its hardness and unmatched color spectrum. Its electrical characteristics have also led to minor industrial applications, such as in pressure gauges and sensitive electronic components.