Tourmaline ~ Mineral of Jun' '19
Mineral of the month - Tourmaline
The name "tourmali" was a generic name used in Ceylon [Sri Lanka] for colored gems, mostly zircons. About 1703, it had been discovered by Dutch lapidaries that some of the "zircons" arriving in the Netherlands were actually a previously undescribed mineral. Tourmalin, as a more or less specific mineral name, was used by Rinmann in 1766. Hill called it Tourmaline Garnet in 1771 and Richard Kirwan shortened the name to "Tourmaline" in 1794.
· Mineral: Tourmaline
· Chemistry: A(D3)G6(Si6O18)(BO3)3X3Z
where A = Ca, Na, K, or is vacant (large cations); D = Al, Fe2+, Fe3+, Li, Mg2+, Mn2+ (intermediate to small cations - in valence balancing combinations when the A site is vacant; G = Al, Cr3+, Fe3+, V3+ (small cations); Si can sometimes have minor Al and/or B3+ substitution.
X = O and/or OH
Z = F, O and/or OH.
· Class: Cyclosilicate (Trigonal borosilicate)
· Crystal system: Hexagonal
· Color: Most commonly black, but can range from colorless to brown, red, orange, yellow, green, blue, violet, pink, or hues in between; can be bi-colored, or even tri-colored; rarely can be neon green or electric blue
· Refractive index: 1.610 – 1.675
· Luster: Vitreous, sometimes resinous
· Specific gravity: 2.9 - 3.1
· Mohs Hardness: 7
· Cleavage: Indistinct
· Fracture: Uneven, small conchoidal, brittle
Due to the vast combinations of elements that can occupy positions in the crystal lattice, Tourmaline is often referred to as a garbage can (or kitchen sink) mineral. Thirty-three "pure" combinations have been identified and given specific names as varieties of the Tourmaline group by the International Mineralogical Association. There is also a list of trade names for Tourmaline based upon color or color combinations ("color name")..
It can be impractical to impossible to determine the chemical composition of the various specimens in the field, classroom, office, or laboratory, hence the generic name "Tourmaline" is used for any mineral in the tourmaline group.
One factor in Tourmaline's popularity as a gemstone is that it occurs in every color - it has one of the widest color ranges of any gem species. To aid in discussions between the public and jewelers or gemologists a trade name based on the color of tourmaline is used:
Schorl black; Sodium/Iron rich; most common species of tourmaline, 95% or more of all tourmaline in nature
Elbaite Sodium/Lithium rich; offers the widest range of gem tourmaline colors
red or pinkish red - Rubellite (Lithium rich)
light blue to bluish green - Indicolite
green - Verdelite
pink core & green rim - Watermelon
colorless - Achroite
Liddicoatite blue; Calcium/Lithium rich
Dravite brown tourmaline; Sodium/Magnesium rich
Paraiba intense greenish/bluish; colored by presence of Copper
Physical properties of Tourmaline useful in identification of the mineral include
* a prismatic crystal habit typically with striations parallel to the long axis of the crystal
* a triangular or 6-sided cross-section with rounded edges
* often color zones through their cross-section or along their length
* can be pleochroic - darkest when viewed down the c-axis, lighter when view perpendicular to the c-axis
* indistinct cleavage
Tourmaline is a six-member ring cyclosilicate having a trigonal crystal system. It occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section, often with curved striated faces. The style of termination at the ends of crystals can be different at opposite ends, called hemimorphism. All hemimorphic crystals are piezoelectric, and are often pyroelectric as well
Tourmaline is a common constituent of granite and granite pegmatites, and can be found in hydrothermal veins and in metamorphic rocks. Schorl and lithium-rich tourmalines are usually found in granite and granite pegmatite. Magnesium-rich tourmalines are generally restricted to schists and marble. Due to its hardness and relative resistance to chemical weathering, Tourmaline can be transported by water long distances from their source area as alluvial deposits. Tourmaline typically occurs as an accessory mineral. Large, well-formed crystals of tourmaline can form in cavities and fractures during hydrothermal activity.
The primary use of tourmaline in general is as a semi-precious gemstone. Tourmaline is a birthstone for the month of October and the gemstone of the 8th Anniversary.
Gem and specimen tourmaline is mined chiefly in Brazil (the world's leading source of tourmaline for nearly 500 years) and Africa. Some placer material suitable for gem use comes from Sri Lanka. In addition to Brazil, tourmaline is mined in Tanzania, Nigeria, Kenya, Madagascar, Mozambique, Namibia, Afghanistan, Pakistan, Sri Lanka, and Malawi. In the United States, fine gems and specimen materials have been produced in Maine and San Diego County, California.
Some of the locales from which specimens of Tourmaline are available at Taos Rockers are:
Melody, in her book Love Is In The Earth, says of Tourmaline: "The energy of Tourmaline relates to each of the chakras. It acts to clear, to maintain, and to stimulate each of the energy center of the body...It is thought to bring healing powers to the user and to provide protection for all dangers occurring in the physical plane.".