Editing Tourmaline (section)

== Physical properties ==
=== Crystal structure ===
[[File:tourmaline01.jpg|thumb|upright|Tri-chromatic elbaite crystals on quartz, Himalaya Mine, San Diego Co., California, US]]

Tourmaline is a six-member ring [[cyclosilicate]] having a [[trigonal]] crystal system. It occurs as long, slender to thick prismatic and columnar [[crystal]]s that are usually triangular in cross-section, often with curved striated faces. The style of termination at the ends of crystals is sometimes asymmetrical, called hemimorphism. Small slender prismatic crystals are common in a fine-grained [[granite]] called [[aplite]], often forming radial daisy-like patterns. Tourmaline is distinguished by its three-sided prisms; no other common mineral has three sides. Prisms faces often have heavy vertical striations that produce a rounded triangular effect. Tourmaline is rarely perfectly [[euhedral]]. An exception was the fine dravite tourmalines of [[Yinnietharra]], in western Australia. The deposit was discovered in the 1970s, but is now exhausted. All hemimorphic crystals are [[Piezoelectricity|piezoelectric]], and are often [[pyroelectricity|pyroelectric]] as well.{{citation needed|date=September 2018}}

A crystal of tourmaline is built up of units consisting of a six-member silica ring that binds above to a large cation, such as sodium. The ring binds below to a layer of metal ions and hydroxyls or halogens, which structurally resembles a fragment of [[kaolin]]. This in turn binds to three triangular borate ions. Units joined end to end form columns running the length of the crystal. Each column binds with two other columns offset one-third and two-thirds of the vertical length of a single unit to form bundles of three columns.  Bundles are packed together to form the final crystal structure. Because the neighboring columns are offset, the basic structural unit is not a [[unit cell]]: The actual unit cell of this structure includes portions of several units belonging to adjacent columns.<ref>{{cite journal |first1=Gabrielle E. |last1=Hamburger |first2=M.J. |last2=Buerger |title=The structure of tourmaline |journal=American Mineralogist |year=1948 |volume=33 |number=9–10 |pages=532–540 |url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/33/9-10/532/538799 |access-date=15 February 2021}}</ref><ref>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |pages=303–304}}</ref>
<gallery>
File:Tourmaline oblique single.jpg|Oblique view of a single unit of the tourmaline crystal structure.
File:Tourmaline 001 single.png|View of single unit of tourmaline structure along the axis of the crystal
File:Tourmaline 100 3.png|View along a axis of three columns of tourmaline units forming a bundle
File:Tourmaline 001 mass.png|Structure of a tourmaline crystal viewed looking along the c axis of the crystal
</gallery>

=== Color ===
[[File:Three tourmalines flash III.jpg|thumb|Two dark-green rectangular tourmaline stones and one oval tourmaline stone]]
[[File:tourmaline.xtal.750pix.jpg|thumb|upright|Bi-chromatic tourmaline crystal, {{convert|0.8|in|cm|0}} long]]
[[File:Tourmaline Mineral Macro 2.JPG|thumb|Tourmaline mineral, approximately {{cvt|10|cm}} tall]]

Tourmaline has a variety of colors. Iron-rich tourmalines are usually black to bluish-black to deep brown, while magnesium-rich varieties are brown to yellow, and lithium-rich tourmalines are almost any color: blue, green, red, yellow, pink, etc. Rarely, it is colorless. Bi-colored and multicolored crystals are common, reflecting variations of fluid chemistry during crystallization. Crystals may be green at one end and pink at the other, or green on the outside and pink inside; this type is called {{visanc|Watermelon tourmaline|text=[[watermelon]] tourmaline}} and is prized in jewelry.  An excellent example of watermelon tourmaline jewelry is a brooch piece (1969, gold, watermelon tourmaline, diamonds) by [[Andrew Grima]] (British, b. Italy, 1921–2007), in the collection of Kimberly Klosterman and on display at the [[Cincinnati Art Museum]].<ref>{{Cite web|date=2022-01-19|title=Simply Brilliant – An Exceptional Collection of Fine Jewelry with Outstanding Stones and Crystals {{!}} GeoRarities|url=https://georarities.com/2022/01/19/simply-brilliant-an-exceptional-collection-of-fine-jewelry-with-outstanding-stones-and-crystals/|access-date=2022-01-27|language=en-US}}</ref> Some forms of tourmaline are [[Dichroism|dichroic]]; they change color when viewed from different directions.<ref>{{Cite web|title=Tourmaline {{!}} mineral|url=https://www.britannica.com/science/tourmaline-mineral|access-date=2021-06-03|website=Encyclopedia Britannica|language=en}}</ref>

The pink color of tourmalines from many localities is the result of prolonged natural irradiation. During their growth, these tourmaline crystals incorporated [[Manganese|Mn<sup>2+</sup>]] and were initially very pale. Due to natural [[gamma ray]] exposure from [[radioactive decay]] of [[Potassium-40|<sup>40</sup>K]] in their [[granitic]] environment, gradual formation of Mn<sup>3+</sup> ions occurs, which is responsible for the deepening of the pink to red color.<ref name="Reinitz & Rossman 1988">Reinitz & Rossman, 1988.</ref>

=== Magnetism ===
Opaque black schorl and yellow tsilaisite are idiochromatic tourmaline species that have high magnetic susceptibilities due to high concentrations of iron and manganese respectively. Most gem-quality tourmalines are of the elbaite species. Elbaite tourmalines are allochromatic, deriving most of their color and magnetic susceptibility from schorl (which imparts iron) and tsilaisite (which imparts manganese).{{Citation needed|date=January 2021}}

Red and pink tourmalines have the lowest magnetic susceptibilities among the elbaites, while tourmalines with bright yellow, green and blue colors are the most magnetic elbaites. Dravite species such as green chromium dravite and brown dravite are diamagnetic. A handheld [[neodymium magnet]] can be used to identify or separate some types of tourmaline gems from others. For example, blue indicolite tourmaline is the only blue gemstone of any kind that will show a drag response when a neodymium magnet is applied. Any blue tourmaline that is diamagnetic can be identified as paraiba tourmaline colored by copper in contrast to magnetic blue tourmaline colored by iron.<ref>[http://gemstonemagnetism.com Kirk Feral ''Magnetism in Gemstones''] {{webarchive|url=https://web.archive.org/web/20131203013148/http://gemstonemagnetism.com/ |date=2013-12-03 }} <!-- WP:RS? --></ref>

=== Treatments ===
Some tourmaline gems, especially pink to red colored stones, are altered by [[heat treatment]] to improve their color. Overly dark red stones can be lightened by careful heat treatment. The pink color in manganese-containing near-colorless to pale pink stones can be greatly increased by irradiation with gamma-rays or electron beams. Irradiation is almost impossible to detect in tourmalines, and does not, currently, affect the value. Heavily included tourmalines, such as rubellite and Brazilian paraiba, are sometimes clarity-enhanced. A clarity-enhanced tourmaline (especially the paraiba variety) is worth much less than an untreated gem of equal clarity.<ref>Kurt Nassau (1984), ''Gemstone Enhancement: Heat, Irradiation, Impregnation, Dyeing, and Other Treatments'', Butterworth Publishers</ref>