{{Infobox mineral | name = Eucryptite | category = Silicate mineral | image = Eucryptite-202043.jpg | imagesize = | caption = Eucryptite grains in albite from the type locality (size: 9.3 × 7.0 × 2.8 cm) | formula = LiAlSiO<sub>4</sub> | IMAsymbol = Ecp<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | system = Trigonal | class = Rhombohedral ({{overline|3}}) <br/>H-M symbol: ({{overline|3}}) | symmetry = ''R''{{overline|3}} | unit cell = a = 13.48, c = 9.01&nbsp;[Å]; Z&nbsp;=&nbsp;18 | color = Brown, colorless, white | habit = Rare as euhedral crystals, coarse crystalline aggregates and massive | cleavage = Indistinct on {10{{overline|1}}0} and {0001} | fracture = Conchoidal | tenacity = Very brittle | mohs = 6.5 | luster = Vitreous | refractive = n<sub>ω</sub> = 1.570 – 1.573 n<sub>ε</sub> = 1.583 – 1.587 | opticalprop = Uniaxial (+) | birefringence = δ = 0.013 | streak = White | density = 2.67 | diaphaneity = Transparent to translucent | other = Fluoresces pink to red or orange under SW UV | references = <ref name=Mindat>[http://www.mindat.org/min-1364.html Mindat.org]</ref><ref name=Handbook>[http://rruff.geo.arizona.edu/doclib/hom/eucryptite.pdf Handbook of Mineralogy]</ref><ref>[https://webmineral.com/data/Eucryptite.shtml Webmineral data]</ref> }}

'''Eucryptite''' is a lithium-bearing aluminium silicate mineral with formula LiAlSiO<sub>4</sub>. It crystallizes in the trigonal - rhombohedral crystal system. It typically occurs as granular to massive in form and may pseudomorphically replace spodumene. It has a brittle to conchoidal fracture and indistinct cleavage. It is transparent to translucent and varies from colorless to white to brown. It has a Mohs hardness of 6.5 and a specific gravity of 2.67. Optically it is uniaxial positive with refractive index values of nω = 1.570 – 1.573 and nε = 1.583 – 1.587.

Its typical occurrence is in lithium-rich pegmatites in association with albite, spodumene, petalite, amblygonite, lepidolite and quartz.<ref name=Handbook/>

It occurs as a secondary alteration product of spodumene. It was first described in 1880 for an occurrence at its type locality, Branchville, Connecticut.<ref name=Mindat/> Its name was from the Greek for ''well concealed'', for its typical occurrence embedded in albite.<ref name=Mindat/><ref name=Handbook/>

==References== {{Reflist}} {{Lithium compounds}} Category:Nesosilicates Category:Lithium minerals Category:Trigonal minerals Category:Minerals in space group 148

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