Tilleyite

Tilleyite
Tilleyite collected from Crestmore quarries, Riverside County, California
General
Category	Sorosilicate
Formula (repeating unit)	Ca3[Si2O7]·2CaCO3
IMA symbol	Tly[1]
Strunz classification	9.BE.82
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	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
Identification
Colour	Colourless, white
Twinning	Simple twinning {100}, often lamellar; α:twin plane ~ 24°
Cleavage	{201} perfect; {100} and {010} poor
Luster	Vitreous, Dull
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.838 - 2.88
Optical properties	Monoclinic (+)
Refractive index	nα = 1.605 - 1.617 nβ = 1.626 - 1.635 nγ = 1.651 - 1.654
Birefringence	δ = 0.035 - 0.046
2V angle	85-89°
Dispersion	r < v
References	[2]

Tilleyite is a rarely occurring calcium sorosilicate mineral with formula Ca₃[Si₂O₇]·2CaCO₃ (sometimes represented as 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.