Editing Calcite (section)

=== Optical ===
[[File:Calcite-refraction-property.jpg|thumb|Photograph of calcite displaying the characteristic birefringence optical behaviour]]
[[File:Fluorescence in calcite.jpg|thumb|Demonstration of birefringence in calcite, using 445&nbsp;nm laser]]
Calcite is [[transparency and translucency|transparent]] to [[opacity (optics)|opaque]] and may occasionally show [[phosphorescence]] or [[fluorescence]]. A transparent variety called "[[Iceland spar]]" is used for optical purposes.<ref>{{cite journal |last1=Harstad |first1=A. O. |last2=Stipp |first2=S. L. S. |title=Calcite dissolution; effects of trace cations naturally present in Iceland spar calcites. |journal=Geochimica et Cosmochimica Acta |date=2007 |volume=71 |issue=1 |pages=56–70 |doi=10.1016/j.gca.2006.07.037 |bibcode=2007GeCoA..71...56H }}</ref> Acute [[bipyramid|scalenohedral]] crystals are sometimes referred to as "dogtooth spar" while the [[rhombohedron|rhombohedral]] form is sometimes referred to as "nailhead spar".<ref name=Sinkankas1964>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0442276249 |pages=359–364}}</ref> The rhombohedral form may also have been the "[[sunstone (medieval)|sunstone]]" whose use by [[Viking]] navigators is mentioned in the [[Sagas of Icelanders|Icelandic Sagas]].<ref>{{cite journal |last1=Ropars |first1=Guy |last2=Lakshminarayanan |first2=Vasudevan |last3=Le Floch |first3=Albert |title=The sunstone and polarised skylight: ancient Viking navigational tools? |journal=[[Contemporary Physics]] |date=2 October 2014 |volume=55 |issue=4 |pages=302–317 |doi=10.1080/00107514.2014.929797 |bibcode=2014ConPh..55..302R |s2cid=119962347 }}</ref>

Single calcite crystals display an optical property called [[birefringence]] (double refraction). This strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. The birefringent effect (using calcite) was first described by the [[Denmark|Danish]] scientist [[Rasmus Bartholin]] in 1669. At a wavelength of about 590&nbsp;nm, calcite has ordinary and extraordinary [[refractive index|refractive indices]] of 1.658 and 1.486, respectively.<ref>{{cite journal |url=http://physics.info/refraction/ |title=Refraction |last=Elert|first=Glenn |journal=The Physics Hypertextbook |year=2021}}</ref> Between 190 and 1700&nbsp;nm, the ordinary refractive index varies roughly between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4.<ref>{{cite journal |doi=10.1016/S0040-6090(97)00843-2| title = Determination of optical anisotropy in calcite from ultraviolet to mid-infrared by generalized ellipsometry| journal=Thin Solid Films| volume=313–314| issue=1–2| pages=341–346| year=1998| last1=Thompson| first1=D. W.| last2=Devries| first2=M. J.| last3=Tiwald| first3=T. E.| last4=Woollam| first4=J. A. |bibcode=1998TSF...313..341T}}</ref>