Tilleyite is a rarely occurring calcium sorosilicate mineral with formula (sometimes represented as ). 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.
{{Infobox mineral | name = Tilleyite | category = Sorosilicate | image = Tilleyite-171531.jpg | imagesize = 260px | caption = Tilleyite collected from Crestmore quarries, Riverside County, California | formula = | IMAsymbol = Tly | strunz = 9.BE.82 | dana = | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/b | unit cell = a = 15.108(3) Å b = 10.241(1) Å c = 7.579(1) Å β = 105.17(1)° 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 = nα = 1.605 - 1.617 nβ = 1.626 - 1.635 nγ = 1.651 - 1.654 | birefringence = δ = 0.035 - 0.046 | pleochroism = | 2V = 85-89° | dispersion = r 1/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.
==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.
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).