Editing Obsidian (section)

==Origin and properties==
[[File:ObsidianDomeCA.JPG|thumb|Obsidian [[Scree|talus]] at [[Obsidian Dome, California]]]]
[[File:Schneeflockenobsidian-Daumenstein.JPG|thumb|Polished snowflake obsidian, formed through the inclusion of [[cristobalite]] crystals]]

The ''[[Natural History (Pliny)|Natural History]]'' by the Roman writer [[Pliny the Elder]] includes a few sentences about a volcanic glass called obsidian (''lapis obsidianus''), discovered in Ethiopia by Obsidius, a Roman explorer.<ref>''[[Dictionary of Greek and Roman Biography and Mythology]]'', [https://archive.org/stream/dictionaryofgree03smituoft#page/2/mode/2up vol. III, p. 2 ("Obsidius")].</ref><ref>[http://www.encyclopedia.com/doc/1O27-obsidian.html obsidian]. The Concise Oxford Dictionary of English Etymology. Oxford University Press (1996). Retrieved November 20, 2011.</ref><ref>D Harper. [http://www.etymonline.com/index.php?allowed_in_frame=0&search=obsidian&searchmode=none obsidian]. Etymology online. June 17, 2012</ref><ref>M H Manser. [https://books.google.com/books?id=ltGvUYNkr8MC&pg=PA342 The Facts On File Dictionary of Allusions]. Infobase Publishing, 2008, {{ISBN|0816071055}}.</ref>

Obsidian is formed from quickly cooled [[lava]].<ref>M E Malainey. [https://books.google.com/books?id=GUlunYOdWvAC&pg=PA150''A Consumer's Guide to Archaeological Science: Analytical Techniques''], Springer, 2010 {{ISBN|1441957030}}</ref><ref name="Svarney">{{cite book|url=https://archive.org/details/isbn_9781578591565_0|url-access=registration|page=[https://archive.org/details/isbn_9781578591565_0/page/123 123]|author1=P L Barnes-Svarney|author2=T E Svarney|publisher=Visible Ink Press|year=2004|title=The Handy Geology Answer Book|isbn=978-1578591565}}</ref><ref name="Società Italiana di Fisica">{{cite book|url=https://books.google.com/books?id=IURum42ob9UC&pg=PA422|author1=M Martini|author2=M Milazzo|author3=M Piacentini|author4=Società Italiana di Fisica|publisher=IOS Press|year=2004|title=Physics Methods in Archaeometry|volume=154|isbn=978-1586034245}}</ref> Extrusive formation of obsidian may occur when [[felsic]] lava cools rapidly at the edges of a felsic lava flow or volcanic dome, or when lava cools during sudden contact with water or air. Intrusive formation of obsidian may occur when felsic lava cools along the edges of a [[dike (geology)|dike]].<ref>{{cite journal|last1=Putnam|first1=William C.|title=The Mono Craters, California|journal=Geographical Review|year=1938|volume=28|issue=1|pages=68–82|doi=10.2307/210567|jstor=210567|bibcode=1938GeoRv..28...68P|s2cid=163772761|url=https://resolver.caltech.edu/CaltechAUTHORS:20191121-125600648}}</ref><ref>{{cite journal|last1=Binder|first1=Didier|last2=Gratuze|first2=Bernard|last3=Mouralis|first3=Damase|last4=Balkan-Atlı|first4=Nur|title=New investigations of the Göllüdağ obsidian lava flows system: a multi-disciplinary approach|journal=Journal of Archaeological Science|date=December 1, 2011|volume=38|issue=12|pages=3174–3184|doi=10.1016/j.jas.2011.05.014|bibcode=2011JArSc..38.3174B}}</ref>

[[Tektite]]s were once thought by many to be obsidian produced by [[moon|lunar]] volcanic eruptions,<ref>{{cite journal|last1=O'Keefe|first1=John A.|title=The Tektite Problem|journal=Scientific American|volume=239|publisher=Munn & Company|year=1978|issue=2|pages=116–127|doi=10.1038/scientificamerican0878-116|jstor=24960359|bibcode=1978SciAm.239b.116O}}</ref> though few scientists now adhere to this [[hypothesis]].<ref>{{cite book|last1=Sevigny|first1=Melissa L.|title=Under desert skies : how Tucson mapped the way to the Moon and planets|date=2016|publisher=Sentinel Peak|location=Tucson|isbn=9781941451045|page=93}}</ref>

Obsidian is mineral-like, but not a true mineral because, as a glass, it is not [[crystal]]line; in addition, its composition is too variable to be classified as a mineral. It is sometimes classified as a [[mineraloid]].<ref name="HandbookSoil">{{cite book|title=Handbook of Soil Sciences: Properties and Processes|publisher=CRC Press|year=2012|isbn=978-1-4398-0306-6|editor-last=Pan Ming Huang|location=Boca Raton|pages=20–24|editor-last2=Yuncong Li|editor-last3=Malcolm E. Sumner|edition=Second}}</ref> Though obsidian is usually dark in color, similar to [[mafic]] rocks such as [[basalt]], the composition of obsidian is extremely felsic. Obsidian consists mainly of SiO<sub>2</sub> ([[silicon dioxide]]), usually 70% by weight or more; the remainder consists of variable amounts of other oxides, mostly oxides of aluminium, iron, potassium, sodium and calcium.<ref name="Liritzis2014">{{cite encyclopedia|last=Liritzis|first=I.|editor1-last=Rink|editor1-first=W.|editor2-last=Thompson|editor2-first=J.|title=Obsidian Hydration Dating|encyclopedia=Encyclopedia of Scientific Dating Methods|url=https://link.springer.com/referenceworkentry/10.1007/978-94-007-6326-5_39-1|year=2014|pages=1–23|publisher=Springer|location=Dordrecht|isbn=978-94-007-6326-5|doi=10.1007/978-94-007-6326-5_39-1}}</ref><ref name="Shackley_2022">{{Cite book|last=Shackley|first=M.S.|title=Obsidian: Geology and Archaeology in the North American Southwest|date=2022|publisher=University of Arizona Press|isbn=9780816550036|pages=12}}</ref> Crystalline rocks with a similar composition include [[granite]] and [[rhyolite]]. Because obsidian is [[Metastability|metastable]] at the Earth's surface (over time the glass [[devitrification|devitrifies]], becoming fine-grained mineral crystals), obsidian older than [[Miocene]] in age is rare. Exceptionally old obsidians include a [[Cretaceous]] welded tuff and a partially devitrified [[Ordovician]] [[perlite]].<ref>{{cite journal|last1=Marshall|first1=Royal R.|title=Devitrification of Natural Glass|journal=GSA Bulletin|date=October 1, 1961|volume=72|issue=10|pages=1493–1520|doi=10.1130/0016-7606(1961)72[1493:DONG]2.0.CO;2|bibcode=1961GSAB...72.1493M}}</ref> This transformation of obsidian is accelerated by the presence of water. Although newly formed obsidian has a low water content, typically less than 1% water by weight,<ref>{{cite web|url=http://www.em.gov.bc.ca/mining/GeolSurv/MetallicMinerals/MineralDepositProfiles/profiles/r12.htm|title=Perlite – Mineral Deposit Profiles|publisher=B.C. Geological Survey|access-date=November 20, 2007|url-status=dead|archive-url=https://web.archive.org/web/20080509024955/http://www.em.gov.bc.ca/mining/GeolSurv/MetallicMinerals/MineralDepositProfiles/profiles/r12.htm|archive-date=May 9, 2008}}</ref> it becomes progressively [[Mineral hydration|hydrated]] when exposed to [[groundwater]], forming [[perlite]].

Pure obsidian is usually dark in appearance, though the color varies depending on the impurities present. Iron and other [[transition element]]s may give the obsidian a dark brown to black color. Most black obsidians contain [[nanoparticle|nanoinclusions]] of [[magnetite]], an [[iron oxide]].<ref name="genesis">{{cite journal|last1=Ma|first1=Chi|last2=Rossman|first2=George|year=2013|title=Nanomineralogy of Gemstones: From Genesis to Discovery|url=https://goldschmidtabstracts.info/2013/1661.pdf|journal=Mineralogical Magazine|volume=77|issue=5|pages=1661|doi=10.1180/minmag.2013.077.5.13|access-date=May 1, 2019|hdl=10174/9676|hdl-access=free}}</ref> Very few samples of obsidian are nearly colorless. In some stones, the [[Inclusion (mineral)|inclusion]] of small, white, radially clustered crystals ([[spherulites]]) of the mineral [[cristobalite]] in the black glass produce a blotchy or snowflake pattern (''snowflake obsidian''). Obsidian may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen (''sheen obsidian''). An [[iridescence|iridescent]], [[rainbow]]-like sheen (''fire obsidian'') is caused by inclusions of [[magnetite]] [[nanoparticle]]s creating [[thin-film interference]].<ref>{{cite journal|last=Nadin|first=E.|year=2007|title=The secret lives of minerals|journal=Engineering & Science|issue=1|pages=10–20|url=http://www.its.caltech.edu/~chima/publications/Secret_Lives_of_Minerals.pdf}}</ref> Colorful, striped obsidian (''rainbow obsidian'') from Mexico contains oriented [[nanorod]]s of [[hedenbergite]], which cause the rainbow striping effects by [[thin-film interference]].<ref name="genesis"/>