Editing Quartz (section)

== Crystal habit and structure ==
{{multiple image
|align=right
|perrow=3
|total_width=450

|image1=Α-Quartz.svg
|caption1=Crystal structure of α-quartz (red balls are oxygen, gray are silicon)

|image2=Β-Quartz.svg
|caption2=Crystal structure of β-quartz

|image3=Alpha-quartz, P3 121 and P3 221.png
|caption3=A chiral pair of α-quartz

}}
Quartz belongs to the [[trigonal crystal system]] at room temperature, and to the [[hexagonal crystal system]] above {{convert|573|C|K F}}. The former is called α-quartz; the latter is β-quartz. The [[crystal habit|ideal crystal shape]] is a six-sided [[prism (geometry)|prism]] terminating with six-sided pyramid-like [[rhombohedron]]s at each end. In nature, quartz crystals are often [[Crystal twinning|twinned]] (with twin right-handed and left-handed quartz crystals), distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear [[Crystal habit#List of crystal habits|massive]].{{sfn|Hurlbut|Klein|1985}}{{sfn|Nesse|2000|p=202–204}}

Well-formed crystals typically form as a [[Druse (geology)|druse]] (a layer of crystals lining a void), of which quartz [[geode]]s are particularly fine examples.<ref name=sinkankas>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0442276249 |pages=443–447}}</ref> The crystals are attached at one end to the enclosing rock, and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, for instance, within [[gypsum]].<ref>{{cite journal|first1=W. A |last1=Tarr |title=Doubly terminated quartz crystals occurring in gypsum |journal=American Mineralogist |year=1929 |volume=14 |number=1 |pages=19–25 |url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/14/1/19/535468 |access-date=7 April 2021}}</ref>

α-quartz crystallizes in the trigonal crystal system, [[space group]] ''P''3<sub>1</sub>21 or ''P''3<sub>2</sub>21 (space group 152 or 154 resp.) depending on the chirality. Above {{convert|573|C|K F}}, α-quartz in ''P''3<sub>1</sub>21 becomes the more symmetric hexagonal ''P''6<sub>4</sub>22 (space group 181), and α-quartz in ''P''3<sub>2</sub>21 goes to space group ''P''6<sub>2</sub>22 (no. 180).<ref>Crystal Data, Determinative Tables, ACA Monograph No. 5, American Crystallographic Association, 1963</ref>

These space groups are truly chiral (they each belong to the 11 enantiomorphous pairs). Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks (SiO<sub>4</sub> tetrahedra in the present case). The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without a change in the way they are linked.{{sfn|Hurlbut|Klein|1985}}{{sfn|Nesse|2000|p=201}} However, there is a significant change in volume during this transition,<ref>{{Cite journal |last1=Johnson |first1=Scott E. |last2=Song |first2=Won Joon |last3=Cook |first3=Alden C. |last4=Vel |first4=Senthil S. |last5=Gerbi |first5=Christopher C. |date=2021-01-01 |title=The quartz α↔β phase transition: Does it drive damage and reaction in continental crust? |url=https://www.sciencedirect.com/science/article/abs/pii/S0012821X20305665 |journal=Earth and Planetary Science Letters |volume=553 |pages=116622 |doi=10.1016/j.epsl.2020.116622 |bibcode=2021E&PSL.55316622J |issn=0012-821X}}</ref> and this can result in significant microfracturing in ceramics during firing,<ref>{{cite journal |last1=Knapek |first1=Michal |last2=Húlan |first2=Tomáš |last3=Minárik |first3=Peter |last4=Dobroň |first4=Patrik |last5=Štubňa |first5=Igor |last6=Stráská |first6=Jitka |last7=Chmelík |first7=František |title=Study of microcracking in illite-based ceramics during firing |journal=Journal of the European Ceramic Society |date=January 2016 |volume=36 |issue=1 |pages=221–226 |doi=10.1016/j.jeurceramsoc.2015.09.004}}</ref> in ornamental stone after a fire<ref>{{Cite journal |last1=Tomás |first1=R. |last2=Cano |first2=M. |last3=Pulgarín |first3=L. F. |last4=Brotóns |first4=V. |last5=Benavente |first5=D. |last6=Miranda |first6=T. |last7=Vasconcelos |first7=G. |date=2021-11-01 |title=Thermal effect of high temperatures on the physical and mechanical properties of a granite used in UNESCO World Heritage sites in north Portugal |url=https://linkinghub.elsevier.com/retrieve/pii/S2352710221006811 |journal=Journal of Building Engineering |volume=43 |pages=102823 |doi=10.1016/j.jobe.2021.102823 |issn=2352-7102|hdl=10045/115630 |hdl-access=free }}</ref> and in rocks of the Earth's crust exposed to high temperatures,<ref>{{cite journal |last1=Johnson |first1=Scott E. |last2=Song |first2=Won Joon |last3=Cook |first3=Alden C. |last4=Vel |first4=Senthil S. |last5=Gerbi |first5=Christopher C. |title=The quartz α↔β phase transition: Does it drive damage and reaction in continental crust? |journal=Earth and Planetary Science Letters |date=January 2021 |volume=553 |pages=116622 |doi=10.1016/j.epsl.2020.116622|bibcode=2021E&PSL.55316622J |s2cid=225116168 |doi-access=free }}</ref> thereby damaging materials containing quartz and degrading their physical and mechanical properties.

<gallery class="center">
File:00026 40 mm quartz.jpg|Common, prismatic quartz
File:Améthyste, quartz 300-3-7640.JPG|Sceptered quartz
File:Quartz sceptres fumés sur quartz (Madagascar) 1.jpg|Sceptered quartz (as aggregates: "Elestial quartz")
File:Quartz-314899.jpg|Bipyramidal quartz
File:Quartz-197980.jpg|Tessin or tapered quartz
File:Hyaline quartz-MCG-NM-IMG 7481-black.jpg|Twinned quartz (known as Japan law)
File:Quartz sur quartz 7(Brésil).jpg|Dauphine quartz (single dominant face)
File:Herkimer.jpg|"[[Herkimer diamond]]"
File:Quartz crystals Macro 1.JPG|Druse quartz
File:Chalcedony (48723879712).jpg|Granular quartz
File:Rose quartz SiO2 locality - Dolní Bory, Czech Republic (50660502442).jpg|Massive quartz
</gallery>