Tourmaline: Mineral Information Page

Links To Tourmaline Information Topics On This Page:
Tourmaline Varieties And Items in our VIRTUAL GALLERY
Physical Properties
Background Info
Occurance and Diagnostic Features
History and Uses
Metaphysical Properties

TOURMALINE PHYSICAL PROPERTIES

• General composition: (Na, Ca)(Li, Mg, Al)(Al, Fe, Mn)₆(BO₃)₃(Si₆O₁₈)(OH)₄
• Chemical Composition of Ideal Endmember Tourmaline Species
  • Buergerite: NaFe₃Al₆B₃Si₆O₃₀F
  • Chromdravite: NaMg₃[Cr,Fe⁺³]₆B₃Si₆O₂₇(OH)₃(OH,F)
  • Dravite: NaMg₃Al₆B₃Si₆O₂₇(OH)₃(OH,F)
  • Elbaite: Na(Li,Al)₃Al₆B₃Si₆O₂₇(OH)₃(OH,F)
  • Feruvite: CaFe⁺²₃Al₆B₃Si₆O₂₇(OH)₃(OH,F)
  • Foitite: [][Fe⁺²₂(Al,Fe⁺³)]Al₆B₃Si₆O₂₇(OH)₃(OH,F)
  • Liddicoatite: Ca(Li,Al)₃Al₆B₃Si₆O₂₇(OH)₃(OH,F)
  • Olenite: NaAl₃Al₆B₃Si₆O₂₇(OH)₃(OH,F)
  • Povondraite: NaFe⁺³₃Fe⁺³₆B₃Si₆O₂₇(OH)₃(OH,F)
  • Schorl: Na(Fe,Mn)₃Al₆B₃Si₆O₂₇(OH)₃(OH,F)
  • Uvite: CaMg₃(Al₅Mg)B₃Si₆O₂₇(OH)₃(OH,F)
• Class: cyclosilicates
• Crystal system: hexagonal-rhombohedral; 3m
• Crystal habit: commonly forms prismatic crystals with well developed trigonal prism and second-order hexagonal prism faces, can be elongated or flattened, commonly vertically striated. In cross section the prism faces often round into one another giving the appearance of a spherical triangle. Doubly terminated crystals often show different forms at each end of the vertical axis. Also radiating, acicular, columnar, massive.
• Twinning: very rare and simple on pyramidal planes such as {1011}
• Specific gravity: 2.96 - 3.31 (buergerite 3.31, dravite 3.03-3.15, elbaite 3.03-3.10, liddicoatite 3.02-3.08, schorl 3.10-3.25 and uvite 2.96-3.06)
• Index of refraction: 1.610-1.735 (buergerite 1.655-1.735, dravite 1.610-1.661, elbaite 1.615-1.655, liddicoatite 1.621-1.637, schorl 1.625-1.675 and uvite 1.612-1.638)
• Birefringence: 0.016-0.080 (buergerite 0.080, dravite 0.021-0.026, elbaite 0.016-0.024, liddicoatite 0.016,schorl 0.025-0.035 and uvite 0.017-0.020)
• Pleochroism: strong in all species
• Hardness: 7.0 - 7.5
• Color: tourmalines occur in all colors. Buergerite is dark brown to black. Dravite is usually brown to black, can be colorless. Elbaite and liddicoatite occur in a wide range of colors and shades including green, yellow, blue, pink to red, colorless and brown. Schorl is usually black, dark blue or dark blue-green. Uvite is usually black, brown or green. Tourmaline crystals are often color zoned concentrically and/or along the C axis.
• Luster: vitreous to resinous, can occur with chatoyancy due to tubular cavities parallel to the C axis.
• Transparency: transparent to opaque
• Cleavage: poor in all species except buergerite which has distinct prismatic cleavage
• Fracture: conchoidal, brittle
• Streak: white

TOURMALINE BACKGROUND INFORMATION

Tourmaline is the name given to a group of related minerals all having essentially the same crystal structure but varying considerably in chemical composition and in some of their physical properties. It is best described as a very complex aluminum borosilicate. There are eleven distinct mineral species of tourmaline based on chemical composition: buergerite, chromdravite, dravite, elbaite, feruvite, foitite, liddicoatite, olenite, povondraite, schorl and uvite.

Tourmaline occurs in all colors. Commonly used names have been given to tourmalines of specific colors or color combinations such as Rubellite for pink to red shades, indicolite for blue tourmaline and watermelon for tourmalines showing concentric color zoning with pink-red cores surrounded by green. Since some species of tourmaline such as elbaite and liddicoatite can look exactly the same and laboratory analysis is necessary to determine the precise tourmaline species, the generally preferred nomenclature today for tourmalines that have not been lab analysed is simply to use the color of the specimen as a prefix followed by tourmaline as in green tourmaline.

The color in tourmaline is due to the presence of metal ions (Fe, Mn, Cr, V, Ti and Cu) in its crystal structure. The color we see in tourmalines is produced when light is absorbed by these ions or by interactions between these ions (intervalent charge transfers).

Some tourmalines form with inclusions of thread-like tubes or cavities which usually occur parallel to the length of the crystal. If these inclusions occur in sufficient number, they may produce a chatoyant effect that yields a cat's-eye when cut into a cabochon gem.

OCCURRENCE AND DIAGNOSTIC FEATURES

Tourmaline forms in a variety of geologic settings. It occurs most often in granite pegmatites and in their immediate vicinity in the enclosing host rocks. Pegmatitic tourmaline is commonly black and is associated with microcline, albite, quartz and muscovite. The light colored gem tourmalines are much more rare, usually occuring in pegmatite core zones with quartz, clevelandite, muscovite, lepidolite and more rarely amblygonite and spodumene.

Other occurances for tourmaline are: with quartz in hydrothermal veins where heated mineral bearing liquids or gases from deep igneous sources later cooled and crystallized along rock fractures, in granites due to late stage alteration of micas and feldspars by boron containing fluids, and by boron metasomatism in contact and regionally metamorphosed rocks. Some tourmaline bearing mica schists may have formed by regional metamorphism of argillaceous sediments containing evaporite borates.

Because of tourmaline's relatively high hardness and specific gravity, it is often found in eluvial and aluvial deposits.

Tourmaline is usually recognized by its rounded triangular basal sections, color, lack of cleavage and conchoidal fracture. Transparent specimens can often be distinguished by their strong pleochrism/dichroism.

HISTORICAL INFORMATION AND USES

The name tourmaline is of the Sinhalese turmali which was originally applied to an assortment of colored stones being mainly zircons. Buergerite was named after Professor Martin J. Buerger, a well known research scientist and crystallographer. Dravite was named after the Drave region in Carinthia. Elbaite was named after the isle of Elba in Italy, and liddicoatite was named after Richard T. Liddicoat of the the Gemologial Institute of America. Schorl is an old German mining term for gangue minerals. Uvite is named for the Uva district in Sri Lanka.

Tourmaline is used as a gemstone, occuring in all colors. Tourmaline also has many scientific and technological uses due to the fact that an electrical charge can be induced in some tourmaline crystals simply by applying pressure to the crystal in the direction of the vertical crystal axis. This effect is known as piezoelectricity, and has many uses in pressure measuring equipment and other scientific applications. Some tourmalines also show pyroelectricity, which occurs when the crystal is heated yielding a positive charge at one end of the crystal and a negative charge at the other.

Tourmaline is the birthstone for the month of October.
The astrological sign of tourmaline is Libra.

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METAPHYSICAL PROPERTIES

Tourmalines are said to enhance one's understanding, increase self-confidence and amplify one's psychic energies. They also are said to neutralize negative energies, dispell fear and grief, and to aid in concentration and communication.
Tourmalines are also said to be usefull in relaxing the body and the mind, and to help in the treatment of infectious diseases, anxiety, blood poisoning, arthritis and heart disease.

For more in-depth metaphysical information, see our Metaphysical Books section.

Tourmaline Item Categories:
Select a link below to view our on-line inventory of that category for all tourmaline varieties
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• Tourmaline Mineral Specimens and Crystals
• Tourmaline Gemstones
• Tourmaline Facet Rough
• Tourmaline Jewelry Items and 14k Earrings

This is the end of our Tourmaline Factsheet and Information page.