{{short description|Calcium sorosilicate mineral}} {{Infobox mineral | name = Tilleyite | category = Sorosilicate | image = Tilleyite-171531.jpg | imagesize = 260px | caption = Tilleyite collected from Crestmore quarries, Riverside County, California | formula = {{chem2|Ca3[Si2O7]*2CaCO3}} | IMAsymbol = Tly<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=10.2138/am.2010.3371|url=https://www.gt-crust.ru/jour/article/view/363 |doi-access=|url-access=subscription}}</ref> | strunz = 9.BE.82 | dana = | system = Monoclinic | class = Prismatic (2/m) <br/><small>(same H-M symbol)</small> | symmetry = ''P2''<sub>1</sub>/b | unit cell = a = 15.108(3) Å <br> b = 10.241(1) Å <br> c = 7.579(1) Å <br> β = 105.17(1)° <br> Z = 4 | color = | colour = Colourless, white | habit = | twinning = Simple twinning {100}, often lamellar; α:twin plane ~ 24° | cleavage = {201} perfect; {100} and {010} poor | fracture = | tenacity = | toughness = | mohs = | luster = Vitreous, Dull | streak = White | diaphaneity = Transparent to translucent | gravity = 2.838 - 2.88 | density = | polish = | opticalprop = Monoclinic (+) | refractive = n<sub>α</sub> = 1.605 - 1.617 <br> n<sub>β</sub> = 1.626 - 1.635 <br> n<sub>γ</sub> = 1.651 - 1.654 | birefringence = δ = 0.035 - 0.046 | pleochroism = | 2V = 85-89° | dispersion = r < v | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | Curie temp = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = | prop1 = | prop1text = | references = <ref name=Mindat>[http://www.mindat.org/min-3964.html Tilleyite on Mindat.org]</ref> }}

'''Tilleyite''' is a rarely occurring calcium sorosilicate mineral with formula {{chem2|Ca3[Si2O7]*2CaCO3}} (sometimes represented as {{chem2|Ca5(CaCO3)2[Si2O7]}}). It is chemically a calcium silicate with additional carbonate ions. Tilleyite crystallizes in the monoclinic crystal system and forms only poorly developed, irregularly defined, tabular crystals and spherical grains. In its pure form it is colorless and transparent, however due to multiple refractions of light from lattice defects or polycrystalline formation, it can also appear white, with the transparency decreasing accordingly.

==History== Tilleyite was first described from an occurrence at the Crestmore Quarry in Riverside County, California in 1932 by Esper Larsen and Kingsley Dunham, and named after Cecil Edgar Tilley (1894&ndash;1973), a professor of geology at the University of Cambridge, in recognition of his contributions to the study of metamorphism.<ref>{{cite journal|last1 = Larsen|first1 =Esper S.|last2 = Dunham|first2 = Kingsley C.|date = Nov 1933|title= Tilleyite, a new mineral from the contact zone at Crestmore, California| url = https://pubs.geoscienceworld.org/msa/ammin/article/18/11/469/536735/Tilleyite-a-new-mineral-from-the-contact-zone-at|journal=American Mineralogist|volume=18|issue =11|pages=469–473|access-date=6 June 2023}}</ref> Its type material is held at Harvard University, and the National Museum of Natural History.<ref>{{cite web|url=https://rruff.info/doclib/hom/tilleyite.pdf|title=Tilleyite|publisher=Mineral Date Publishing|date= 2001|access-date=6 June 2023}}</ref>

==Crystal structure== Tilleyite crystallizes monoclinically in the space group ''P''2<sub>1</sub>/a (space group no. 14, position 3) with the lattice parameters a = 15.11 Å; b = 10.24Å; c = 7.58 Å,and β = 105.2°, with 4 formula units per unit cell.<ref>{{cite book| last1= Strunz|first1 = Hugo|last2 = Nickel|first2 = Ernest H.| title= Strunz Mineralogical Tables - Chemical-structural Mineral Classification System |edition = 9 | date = 2001 | isbn= 3-510-65188-X | page= 583| publisher=Schweizerbart }}</ref>

==Formation and occurrence== Tilleyite is formed by contact metamorphism in the zone between volcanics and limestones at low pressure and high temperatures. Associated minerals include calcite, fluorite, gehlenite, grossular, vesuvianite and wollastonite.<ref>{{cite book|first1 = John W. |last1=Anthony |first2=Richard A. |last2=Bideaux |first3=Kenneth W. |last3=Bladh |first4=Monte C. |last4=Nichols|chapter=Tilleyite|date=2001|title = Handbook of Mineralogy |publisher= Mineralogical Society of America|url=https://www.handbookofmineralogy.org/pdfs/tilleyite.pdf}}</ref>

More generally, it occurs at gabbro-limestone contacts, such as at Carlingford, Ireland, and on the island of Muck, Scotland.<ref>{{cite book | title=An Introduction to the Rock-forming Minerals (2nd edition) Volume 4A | publisher=Geological Society of London | last1=Deer | first1=W.A | last2=Howie | first2=R.A. | last3=Zussman | first3=J. | year=2001 | location=London | pages=115 | isbn=1-86239-081-9}}</ref>

==See also== *List of minerals

==External links== *[https://www.mindat.org/min-3964.html Tilleyite - mindat.org]

==References== {{reflist}}

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Category:Calcium minerals Category:Sorosilicates Category:Monoclinic minerals Category:Carbonate minerals Category:Minerals described in 1933