Editing Tourmaline (section)

=== 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>