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16.05: Quartz

  • Page ID
    3274
  • Quartz
    Chemical composition SiO2 Silicon dioxide
    Crystal system Trigonal
    Habit Prismatic
    Cleavage Poor
    Fracture Conchoidal
    Hardness 7
    Optic nature Uniaxial +
    Refractive index 1.544 - 1.553
    Birefringence 0.009
    Dispersion 0.013
    Specific gravity 2.65
    Lustre Vitreous
    Pleochroism None

    Figure \(\PageIndex{1}\): Quartz crystal cluster specimen
    Sinai, Egypt

    Quartz is a common variety of silicon dioxide that often crystallizes into well-formed hexagonal prisms. It occurs in many colors, including purple amethyst, yellow citrine, and a smoky variety, referred to as smoky quartz or cairngorm. Colorless, transparent quartz crystals represent the modern concept of "crystal".

    Crystal balls with a truly mystic significance are always made from quartz. Quartz crystals are among the earliest talismans. Examples of rock crystal objects date back to 75,000BC. Every culture on the planet has attributed magical powers to quartz crystals.
    The Japanese considered it "the perfect jewel", symbolizing patience and perseverance, the infinity of space and purity. Native Americans used quartz crystals as a hunting talisman and "fed" them with deer blood. Crystal balls, credited with the ability to cure diseases, were brought to Europe from the Near East by the Crusaders. In Scotland and Ireland, farmers used crystal balls to keep livestock healthy.

    Diagnostics

    Crystalline quartz (especially rock crystal) can be confused with many other gemstones such as topaz, scapolite, danburite, phenakite, and others. Usually, a refractometer reading will eliminate most of them.
    Scapolite has properties close to quartz, but has a negative optic sign.

    Color

    Diaphaneity

    Transparent to opaque.

    Polariscope

    Only two minerals that are commonly used as gemstones have a uniaxial character with a positive optic sign, quartz, and zircon. Some others are phenakite, cinnabar, synthetic moissanite, and some idocrase.
    In addition, crystalline quartz will usually show a "bulls-eye" due to enantiomorphism. When one encounters a conoscopic interference figure with a bulls-eye, it can be nothing else than quartz.
    Or in the case of quartz with Brazil twinning (as amethyst) it will show quadruple Airy Spirals.

    With the addition of a quarter wave plate one can observe if the specimen is left or right-handed quartz, or both. The double Airy Spirals will either spiral to the left or to the right, indicating left or right-handedness.

    Refractometer

    Quartz will show fairly constant readings with nω = 1.544 and nε = 1.553, the full birefringence is 0.009.
    Optic character: uniaxial +.

    Other minerals falling in this range:

    • Scapolite - Uniaxial with a negative optic sign
    • Iolite - Biaxial optic character
    • Feldspar - Biaxial optic character

    Specific gravity

    The s.g. of quartz is 2.65 and it suspends in bromoform diluted with toluol.

    Magnification

    • "Tiger stripe" inclusions
    • ribbon like reddish inclusions
    • ball or drop-shaped opaque inclusions
    • 2-phase inclusions
    • 3-phase inclusions
    • negative crystal inclusions
    • angular zoning in amethyst, citrine and smokey quartz

    References

    • Quartz (1987) - Michael O'Donoghue ISBN 0408014628

    See also