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16.14: Tourmaline

  • Page ID
    4341
  • Tourmaline
    Chemical composition Complex borosilicate
    Crystal system Trigonal
    Habit Trigonal prisms with convex faces
    Fracture Conchoidal
    Hardness 7.5
    Optic nature Uniaxial -
    Refractive index 1.62 - 1.64
    Birefringence 0.014-0.021 (up to 0.039)
    Dispersion Low, 0.018
    Specific gravity 3.06 (3.01-3.11)
    Lustre Vitreous
    Pleochroism Weak to strong

    File:Bi Color2.jpg

    Figure \(\PageIndex{1}\): Bi-color tourmaline
    Photo courtesy of Lembeck Gems

    Tourmaline image gallery

    Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.

    Tourmaline group

    Tourmaline is a large group consisting of complex borosilicates.

    Species

    Only 5 species of tourmaline are of real importance to gemologists.

    File:DraviteTourmaline.gif

    Figure \(\PageIndex{2}\): Faceted Dravite Tourmaline
    Photo courtesy of The Gem Trader

    • Chromdravite
    • Schorl
      • Schorl is the, iron rich, black variety of the tourmaline group and is used in mourning jewelry.
        Its name comes from the old mining expression for 'false one'.

    From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.

    Varieties

    There are many, mainly color, varieties of these species.

    Color varieties (names apply to all species).

    • Achroite - colorless tourmaline
    • Rubellite - red tourmaline (color due to iron and manganese)
    • Indicolite - blue tourmaline (color due to iron)
    • Verdelite - green tourmaline (color due to iron and titanium)
    • Siberite - reddish-violet tourmaline
    • Watermelon - a pink core with green edges

    File:Watermelon41039ct.jpg

    Figure \(\PageIndex{3}\): Watermelon Tourmaline Slices
    Photo courtesy of Olivier Louy, M.Sc, A.G. (HH)

    Other varieties.

    • Paraiba - neon colored elbaite tourmaline (color due to copper and manganese)

    Diagnostics

    Color

    Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.

    Refractive index

    The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).
    nω = 1.631-1.698, nε = 1.610-1.675, optic sign is negative.
    The indices of refraction increase with higher iron content.

    Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "Kerez effect". Careful recutting of the stone will reveal that it is an outer-edge phenomenon [Dietrich, 1985].

    Polariscope

    Some dark colored tourmalines have a so-called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.
    Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.

    Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].

    Magnification

    Tourmalines can be of type I to type III clarity grades.
    Typical inclusions are:

    • Trichites (small thread-like twists)
    • Flattened liquid channels running parallel to the optic axis.
    • Liquid Veils
    • 2 and 3-Phase Inclusions
    • Hollow tubes

    File:Tourmaline veils tubes.jpg

    Figure \(\PageIndex{4}\): Tourmaline with liquid veils, hollow tubes and phase inclusions
    40X Magnification
    by Barbra Voltaire

    File:Tourmaline xls.jpg

    Figure \(\PageIndex{5}\): Tourmaline filled with negative crystals, oriented in random directions
    80X Magnification
    by Barbra Voltaire

    File:Pink tourm tubes1.jpg

    Figure \(\PageIndex{6}\): Growth Tubes in Pink Tourmaline
    ~75X Magnification
    by Jamey Swisher

    File:Pink tourm tubes2.jpg

    Figure \(\PageIndex{7}\): Ends of Growth Tubes in Pink Tourmaline
    ~75X Magnification
    by Jamey Swisher

    Phenomena

    • Cat's-eyes
    • Color Change

    An Unusual Variety of Color Change Elbaite ("Laurellite")

    A variety of color change tourmaline has been collected and described by Bruce A. Fry.

    File:Bruces reverse cc laurellite tourmaline1.jpg

    Figure \(\PageIndex{8}\): Color Change Tourmaline
    Courtesy of Bruce Fry

    Treatments

    Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.

    A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.
    A deep red Nigerian stone in the gallery below has been heat-treated several times as an experiment by Roger Dery with the ramping done last (following instructions by Lisa Elser on the latter part).

    File:RedTourmalineOval5 32ct.jpg

    Figure \(\PageIndex{9}\): Nigerian red tourmaline. Heat treated at 360° C for 2 hours

    File:RedTourmalineOval5 32v02.jpg

    Figure \(\PageIndex{10}\): Heat treated at 410° C for 2 hours (same stone)

    File:PinkTourmalineOval5 32ct.jpg

    Figure \(\PageIndex{11}\): Ramped up to 520° C over a three hour period. Held at 520° C for two hours, followed by cooling (same stone)


    Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.
    Cobalt-60 gamma irradiation gives rise to pink and hot pink colors in some tourmaline.

    Gallery of Tourmaline Crystals from Paraiba, Brazil

    The following crystals were collected by Jason Barrett.
    All specimens are from Sao Jose da Batalha mine in Paraiba State, Brazil.
    Not all tourmalines from this mine were copper-bearing and these specimens have not been tested for copper content.

    File:Par 1.jpg

    Figure \(\PageIndex{12}\): Crystal #1
    Weight: 88.4 grams
    Measurements:5cm x 4cm x 3cm

    File:Par 1a.jpg

    Figure \(\PageIndex{13}\): Crystal #1
    Weight: 88.4 grams
    Measurements:5cm x 4cm x 3cm

    File:Par 2.jpg

    Figure \(\PageIndex{14}\): Crystal #2
    Weight: 314.1 grams
    Measurements:7cm x 5.5cm x 4.5cm

    File:Par 2a.jpg

    Figure \(\PageIndex{15}\): Crystal #2
    Weight: 314.1 grams
    Measurements:7cm x 5.5cm x 4.5cm

    File:Par 3.jpg

    Figure \(\PageIndex{16}\): Crystal #3
    Weight: 268.8 grams
    Measurements:10cm x 6cm x 4.5cm

    File:Par 3a.jpg

    Figure \(\PageIndex{17}\): Crystal #3
    Weight: 268.8 grams
    Measurements:10cm x 6cm x 4.5cm

    File:Par 3b.jpg

    Figure \(\PageIndex{18}\): Crystal #3
    Weight: 268.8 grams
    Measurements:10cm x 6cm x 4.5cm

    File:Par 4.jpg

    Figure \(\PageIndex{19}\): Crystal #4
    Weight: 81.8 grams
    Measurements:7cm x 4cm x 3cm

    File:Par 4a.jpg

    Figure \(\PageIndex{20}\): Crystal #4
    Weight: 81.8 grams
    Measurements:7cm x 4cm x 3cm

    File:Par-5.jpg

    Figure \(\PageIndex{21}\): Crystal #5
    Weight: 18.9 grams
    Measurements:4.5cm x 2cm x 1cm

    File:Par-5a.jpg

    Figure \(\PageIndex{22}\): Crystal #5
    Weight: 18.9 grams
    Measurements:4.5cm x 2cm x 1cm

    File:Par-6.jpg

    Figure \(\PageIndex{23}\): Crystal #6
    Weight: 3.0 grams
    Measurements:2cm x 1.5cm x 0.5cm

    File:Par-6a.jpg

    Figure \(\PageIndex{24}\): Crystal #6
    Weight: 3.0 grams
    Measurements:2cm x 1.5cm x 0.5cm

    File:Par-7.jpg

    Figure \(\PageIndex{25}\): Crystal #7
    Weight: 24.4 grams
    Measurements:3cm x 3.5cm x 2.5cm

    File:Par 7a.jpg

    Figure \(\PageIndex{26}\): Crystal #7
    Weight: 24.4 grams
    Measurements:3cm x 3.5cm x 2.5cm

    File:Par-8.jpg

    Figure \(\PageIndex{27}\): Crystal #8
    Weight: 52.9 grams
    Measurements:5.5cm x 3.5cm x 2.5cm

    File:Par-8a.jpg

    Figure \(\PageIndex{28}\): Crystal #8
    Weight: 52.9 grams
    Measurements:5.5cm x 3.5cm x 2.5cm

    File:Par 9.jpg

    Figure \(\PageIndex{29}\): Crystal #9
    Weight: 54.8 grams
    Measurements:7cm x 4cm x 1.5cm

    File:Par-9a.jpg

    Figure \(\PageIndex{30}\): Crystal #9
    Weight: 54.8 grams
    Measurements:7cm x 4cm x 1.5cm

    File:Blue tourm barrett.jpg

    Figure \(\PageIndex{31}\): Parcel of Tourmaline
    Courtesy Jason Barrett


    The parcel above was submitted by Jason Barrett for a microprobe analysis by UNO and the MP2 research group.
    The results are as follows:
    SiO2:36.712%
    Al2O3:42.221%
    MnO:1.56%
    CuO:1.471%
    FeO:0.022%
    MgO:0.000%
    Na2O:2.365:
    K2O:0.022%
    F:1.182%
    The 1.471 wt. % of CuO is just right in the range for the true Paraiba (Brazil) material

    Sources