Difference between revisions of "Tourmaline"

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{{tourmaline}}
 
{{tourmaline}}
 
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" />
 
[[Image:Bi_Color2.jpg|left|thumb|300px|Bi-color tourmaline<br />Photo courtesy of Lembeck Gems]]<br clear="left" />
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 east. 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. <br>
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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 east. 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.
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==Diagnostics==
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===Color===
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Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.
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===Refractive index===
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The refractive index of tourmaline lies between 1.610 and 1.698 with a birefringence up to 0.039 (usually 0.019).<br />
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nω = 1.631-1.698, nε = 1.610-1.675, optic sign is negative.<br />
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The indices of refraction increase with higher iron content.
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Probably due to thermal shock, some stones may show 4 (or even 6) different values per reading. This effect is sometimes seen in green tourmalines and is named the "[[Refractometer#Kerez_effect|Kerez effect]]".
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===Polariscope===
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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.<br />
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Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.
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Some tourmalines show pseudo biaxial interference figures on lateral rotation (with a 2V up to 10°).
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===Magnification===
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Tourmalines can be of type I to type III clarity grades.<br />
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Typical inclusions are:
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* Trichites (small thread-like twists)
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* Flattened liquid channels running parallel to the optic axis.
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'''Tourmaline Group:'''<br>
 
'''Tourmaline Group:'''<br>
 
Paraiba Tourmaline<br>
 
Paraiba Tourmaline<br>

Revision as of 14:38, 12 May 2007

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
Bi-color tourmaline
Photo courtesy of Lembeck Gems

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 east. 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.

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 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, some stones may show 4 (or even 6) different values per reading. This effect is sometimes seen in green tourmalines and is named the "Kerez effect".

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 interference figures on lateral rotation (with a 2V up to 10°).

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.



Tourmaline Group:
Paraiba Tourmaline
Rubellite Tourmaline
Indicolite Tourmaline
Verdite Tourmaline
Dravite Tourmaline
Achroite Tourmaline
Chrome Tourmaline
Bicolor Tourmaline
Tricolor Tourmaline
Watermelon Tourmaline
Schorl Tourmaline