Tilleyite
Tilleyite collected from Crestmore quarries, Riverside County, California
General
CategorySorosilicate
Formula
(repeating unit)
Ca3[Si2O7]·2CaCO3
IMA symbolTly[1]
Strunz classification9.BE.82
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/b
Unit cella = 15.108(3) Å
b = 10.241(1) Å
c = 7.579(1) Å
β = 105.17(1)°
Z = 4
Identification
ColourColourless, white
TwinningSimple twinning {100}, often lamellar; α:twin plane ~ 24°
Cleavage{201} perfect; {100} and {010} poor
LusterVitreous, Dull
StreakWhite
DiaphaneityTransparent to translucent
Specific gravity2.838 - 2.88
Optical propertiesMonoclinic (+)
Refractive indexnα = 1.605 - 1.617
nβ = 1.626 - 1.635
nγ = 1.651 - 1.654
Birefringenceδ = 0.035 - 0.046
2V angle85-89°
Dispersionr < 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.

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 Cambridge University, 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

References

  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. doi:10.2138/am.2010.3371.
  2. Tilleyite on Mindat.org
  3. Larsen, Esper S.; Dunham, Kingsley C. (Nov 1933). "Tilleyite, a new mineral from the contact zone at Crestmore, California". American Mineralogist. 18 (11): 469–473. Retrieved 6 June 2023.
  4. "Tilleyite" (PDF). Mineral Date Publishing. 2001. Retrieved 6 June 2023.
  5. Strunz, Hugo; Nickel, Ernest H. (2001). Strunz Mineralogical Tables - Chemical-structural Mineral Classification System (9 ed.). Schweizerbart. p. 583. ISBN 3-510-65188-X.
  6. Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (2001). "Tilleyite" (PDF). Handbook of Mineralogy, Mineralogical Society of America.
  7. Deer, W.A; Howie, R.A.; Zussman, J. (2001). An Introduction to the Rock-forming Minerals (2nd edition) Volume 4A. London: Geological Society of London. p. 115. ISBN 1-86239-081-9.
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