Editing Topaz (section)

== Structure ==
[[File:Topaz.GIF|thumb|Topaz's crystal structure using polyhedrons showcasing the aluminum octahedron (grey), silica tetrahedron (blue), oxygen (red), and fluorine (green).]]
Topaz is an accessory mineral to felsic igneous, sedimentary, and hydrothermally altered rocks.<ref name=":3">{{Cite book |last=Nesse |first=William D. |title=Introduction to mineralogy |date=2017 |publisher=Oxford University Press |isbn=978-0-19-061835-3 |edition=3rd |location=New York Oxford}}</ref>

The crystal structure of topaz alternates between sheets of (F, OH)<sub>2</sub>O and O along (010) with Al<sup>3+</sup> occupying the octahedral sites and Si<sup>4+</sup> in the tetrahedral sites.<ref name=":3" /> Fluorine can be substituted by hydroxide in topaz by up to 30&nbsp;mol.% in nature and hydroxide-dominating topaz can be made in laboratories but has not been found in nature.<ref name=":4">{{Cite journal |last=Wise |first=Michael A. |date=1995–2002 |title=Topaz: A Mineralogical Review |url=http://www.tandfonline.com/doi/abs/10.1080/00357529.1995.9926593 |journal=Rocks & Minerals |language=en |volume=70 |issue=1 |pages=16–25 |doi=10.1080/00357529.1995.9926593 |issn=0035-7529}}</ref>

On occasion, cavities can be found within topaz and they are filled with a liquid called brewsterlinite.<ref name=":1" /> Brewsterlinite was discovered by [[David Brewster]] upon heating a sample of topaz.<ref>{{Cite EB9 |wstitle= Topaz |volume= XXIII |page=446 |short= 1}}</ref> After heating, the topaz lost mass, and through examination Brewster concluded Topaz was formed in a wet environment creating these liquid-filled cavities. This liquid is a hydrocarbon with a refractive index of 1·13.<ref name=":1" />

Topaz's crystal habit takes many forms. It can display a range of slender and long crystals to bulky and short.<ref name=":4" /> There can also be variation in the terminations displaying blunt, pyramidal, chisel, or wedge-shaped terminations. The perfect cleavage {001} in topaz breaks no Si-O bonds within its structure and only breaks Al-O and Al-F bonds.<ref>{{Cite book |last=Ribbe |first=P. H. |url=http://dx.doi.org/10.2138/am-2019- |title=The crystal structure of topaz and its relation to physical properties |last2=Gibbs |first2=G. V. |publisher=Mineralogical Society of America |year=1971 |series=The American mineralogist |volume=56 |location=Washington, DC |pages=24–30 |issn=0003-004X}}</ref> This cleavage is diagnostic for this mineral. The 2V optical angle in topaz can range from 48° to 69.5°.<ref name=":4" /> Low fluorine content yields a smaller angle and high fluorine content yields a larger angle.