Tilleyite

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.

Tilleyite
Tilleyite collected from Crestmore quarries, Riverside County, California
General
Category	Sorosilicate
Formula	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]

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 (1894–1973), a professor of geology at the University of Cambridge, in recognition of his contributions to the study of metamorphism.^[3] Its type material is held at Harvard University, and the National Museum of Natural History.^[4]

Crystal Structure

Tilleyite crystallizes monoclinically in the space group P2₁/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.^[5]

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.^[6]

More generally, it occurs at gabbro-limestone contacts, such as at Carlingford, Ireland, and on the island of Muck, Scotland.^[7]

See also

• List of minerals

External links

• Tilleyite - mindat.org