Tourmaline

By: Ian Hull, Jarettsy Verduzco, & Emma Prine

Mineral Name: Tourmaline

Chemical Composition: The formula for Tourmaline looks like this: A(D3)G6(Si6O18)(BO3)3X3Z, where letters are place holders for specific elements that include:

A = large cations , D = intermediate to small cations, G = small cations, X = O or OH, and Y = F, O, or OH.

Color: The most common color of tourmaline is black. Although, it can also be green, blue, yellow, pink, red, brown, purple, orange, or maybe even colorless. Occasionally the blue can be so deep and dark that it appears to be black. (2)

Streak: White/Colorless

Hardness: 7-7.5

Cleavage/Fracture: The cleavage is most often described as “indistinct” (1). The mineral’s fracture formation is very small, uneven or scattered, often conchoidal (meaning it breaks along a smooth, curved plain), and can also be very brittle.

Crystal Form: Tourmaline has a hexagonal crystal system, meaning that it’s crystals contain a total of 4 axes, three of which are the same length and lie at an angle of 120 degrees from each other (4) . The fourth can be either longer or shorter than the others, but it always lays at a right angle toward the axis (4). Tourmaline has a prismatic crystal form, which is a crystal with four or more sides of equal length (3). Prismatic crystals are often elongated in one direction. Tourmaline can also have a tabular crystal form, but it is much less common. When it does have a tabular crystal form, it appears thin and has 4 sides (9). This mineral is often found in a type of aggregate, which can include columnar (slender, compact, parallel, and adjoining crystals), radiating (either flat or three-dimensional slender crystals compacted together that radiate from a central point), botryoidal (smooth, rounded surfaces or bubbles that look like a cluster of grapes), and stalactic (resembles the icicle formation that gives it its name) (10; 11; 12; 13).

Luster: Tourmaline’s luster is Vitreous

Special features: Tourmaline is a pleochroic mineral (1). This means that the color of the mineral has the potential to change simply by observing it from a different angle. When you view a more translucent sample from the end, it appears darker than when you view it from the side.

Varieties: There are many different varieties of Tourmaline, including (3):

  • Achroite: This variety is clear and occasionally fibrous

  • Cat’s Eye Tourmaline: This is a common name for any variety of tourmaline that displays chatoyancy (a trait where a gem has a band of bright, reflected light. This is caused by alignment incursions in a gemstone)

  • Chrome Tourmaline - This is a chromium-bearing variety of tourmaline

  • Mn-Tourmaline - This is an manganese-bearing variety of tourmaline

  • Mohrenkopf Turmalin - This variety has blackish crystal termination and is pale in color (that is the German word, the English translation is ‘Moor’s Head Tourmaline’)

  • Nikeloan Tourmaline - This is a nickel-bearing variety of tourmaline

  • Rubellite - This variety is lithium-rich, which gives it a red/pinkish color

  • Trapiche Tourmaline - A unique variety that displays six-spoked growth features

  • Vanadium Tourmaline - This is a vanadium-bearing variety of tourmaline

  • Verdelite - A variety of tourmaline that is green in color

  • Watermelon Tourmaline - True to its name, this variety has a pink colored center with green on both sides

Mineral Group: Tourmaline falls into the silicates group because it contains elements of silicon and oxygen (3). It also belongs in the cyclosilicates, which is a group within the silicates.This specific group has tetrahedrons linked into rings (3).


Environment: Tourmaline is typically found as an accessory mineral in metamorphic rocks and igneous rocks. An accessory mineral is a mineral found in a small amount that isn’t used in identification of a rock. Tourmaline is also found in pegmatites, which are thought to form at the end of a cooling magma when leftover materials, referred to as volatiles, that are not easily incorporated into other, more common minerals formed earlier (17). This allows for less common minerals, like tourmaline, to come together to make a new mineral (16). Some tourmaline can also be found in rhyolite lavas (3).

Associated Rock types: Lithium-rich tourmalines are most commonly found in granite and granite pegmatite. Magnesium-rich tourmalines are usually only found in schists and marble. However, tourmaline can also be found as grains in both sandstone and conglomerate (6). This is due to the mineral’s durability, which allows it to survive river and beach transport (14).

Occurrence in North America: In the United states, there has been tourmaline found in southern California mines.Tourmaline has been mined there since the late 1800’s (14). The very first commercial mine was put in use after an 1821 discovery of the mineral in Paris, Maine (14). Many other New England states also have locations where tourmaline can be found, specifically in granite bearing states.

Economic Uses: The mineral is typically a very popular gem that has the potential to be worth a lot of money. Each different form of tourmaline brings not only a new look and color, but also a new price. This is due to the rarity of the different varieties. High quality gems can be up to $1000 a carat (19). While the more common black samples can be found for under $5. Tourmaline crystals are cut into many different shapes and sizes and are represented by many different colors as well. The gem is used for many forms of jewelry and the special colors that it provides make it unique and desired for many collectors. Due to all of the different colors and how the colors change on different sides, different cuts can compliment different things. A cut may blend colors or isolate them. So cutting tourmaline can be difficult due to figuring out what colors would be more desired.

Industrial Uses: Tourmaline is a piezoelectric mineral, meaning that it generates an electrical charge when under stress (7). This makes it very useful in high pressure gauges, an affordable way to measure the pressure of a gas or liquid. The more pressure the gas or liquid puts on the tourmaline, the higher the electrical charge that it will give off. Pressure gauges are among the most common instruments used in industrial plants (8).

First Notable Identification: This mineral had several names before it came down to just tourmaline. There was tourmali which was used to name colored gems in southern Asia (3). In 1703, the dutch discovered some tourmaline (an unnamed mineral) were among the zircon coming into the Netherlands (3). Tourmaline was the name used by a man named Rinmann in 1766 (3). In 1771, the phrase used to describe this mineral was tourmaline garnet (3). Ultimately it came down to Richard Kirwan who decided on tourmaline in 1794 (3).

How We Identified It: With watermelon tourmaline, the unique change from red, clear, to green colors helped greatly to show this type. The visible striations along the black tourmaline we found to be a unique factor in identification. Striations in other minerals tend to be finer or larger. Like beryl, which has striations similar to tourmaline; although, they are finer in size.

Don’t Confuse It With: While Tourmaline is a pretty unique mineral, there are still a few types of minerals that it can be confused with. For example, Beryl can be distinguished from tourmaline by its different striations (which are horizontal and finer), and its crystal form (which is usually more hexagonal). Apatite can also be distinguished using striations (which apatite does not have), and apatite’s hardness (which is only a 5 on Mohs scale). Epidote is another mineral that can be confused for tourmaline but it is softer than tourmaline (about a 6), and has different crystal habits (crystals grow in all different directions in one area) (7). Finally, Hornblende is also softer than tourmaline (anywhere from 5-6 on the hardness scale), and, like apatite, can be distinguished by its lack of striations (15).


Bibliography:

  1. “Tourmaline.” Geology, geology.com/minerals/tourmaline.shtml.

  2. Besler, Carol. “Tourmaline.” Headlines, www.gemstone.org/education/gem-by-gem/122-tourmaline.

  3. “Tourmaline.” Tourmaline: Mineral Information, Data and Localities., www.mindat.org/min-4003.html.

  4. “Hexagonal - Minerals.net Glossary of Terms.” Hexagonal - Minerals.net Glossary of Terms, www.minerals.net/mineral_glossary/hexagonal.aspx.

  5. “Black Tourmaline Meanings and Uses.” Crystal Vaults, https://www.crystalvaults.com/ crystal-encyclopedia/black-tourmaline

  6. “Tourmaline.” Wikipedia, Wikimedia Foundation, 20 Jan. 2020, en.wikipedia.org/wiki/Tourmaline.

  7. “Tourmaline: The Mineral Tourmaline Information and Pictures.” Tourmaline: The Mineral Tourmaline Information and Pictures, www.minerals.net/mineral/tourmaline.aspx.

  8. “OMEGA Engineering.” Omega Engineering, www.omega.co.uk/prodinfo/pressure-gauges.html.

  9. “Tabular - Minerals.net Glossary of Terms.” Tabular - Minerals.net Glossary of Terms, www.minerals.net/mineral_glossary/tabular.aspx.

  10. “Columnar - Minerals.net Glossary of Terms.” Columnar - Minerals.net Glossary of Terms, www.minerals.net/mineral_glossary/columnar.aspx.

  11. “Radiating - Minerals.net Glossary of Terms.” Radiating - Minerals.net Glossary of Terms, www.minerals.net/mineral_glossary/radiating.aspx.

  12. “Botryoidal - Minerals.net Glossary of Terms.” Botryoidal - Minerals.net Glossary of Terms, www.minerals.net/mineral_glossary/botryoidal.aspx.

  13. “Stalactitic - Minerals.net Glossary of Terms.” Stalactitic - Minerals.net Glossary of Terms, www.minerals.net/mineral_glossary/stalactitic.aspx.

  14. “Tourmaline.” Geology, geology.com/minerals/tourmaline.shtml#occurrence.

  15. “Hornblende: The Amphibole Mineral Hornblende Information and Pictures.” Hornblende: The Amphibole Mineral Hornblende Information and Pictures, www.minerals.net/mineral/hornblende.aspx.

  16. “Pegmatite.” Encyclopedia.com, https://www.encyclopedia.com/earth-and-environment/ geology-and-oceanography/geology-and-oceanography/pegmatite

  17. Shannon Othus Gault, personal communication, 2019.

  18. “Tourmaline Pricing Guide.” Tourmaline Pricing Guide at AJS Gems, www.ajsgem.com/articles/tourmaline-pricing-guide.html.