Cristobalite

Cristobalite
Cristobalite
	Cristobalite spherulites formed by devitrification from the obsidian matrix.; Specimen from California, US; size: 5.9 cm × 3.8 cm × 3.8 cm (2.3 in × 1.5 in × 1.5 in).
General
Category	Oxide mineral, quartz group
Formula; (repeating unit)	SiO2
IMA symbol	Crs
Strunz classification	4.DA.15
Dana classification	75.1.1.1
Crystal system	Tetragonal
Crystal class	Trapezohedral (422)
Space group	P41212, P43212
Unit cell	a = 4.9709(1) Å, ; c = 6.9278(2) Å; ; Z = 4 (α polytype)
Structure
Jmol (3D)	Interactive image
	SMILES O[Si]3(O)O[Si](O)(O[Si]4(O)O)O[Si](O)(O)O[Si](O0)(O)O[Si]1(O)O[Si]5(O3)O[Si]2(O)O[Si](O4)(O)O[Si]0(O)O[Si](O)(O)O[Si](O)(O[Si](O)(O)O1)O[Si](O)(O2)O[Si](O)(O)O[Si](O)(O)O[Si](O)(O)O5(SMILES input only shows in Preview. SMILES is used to create the Jmol 3D image.)
Identification
Color	Colorless, white
Crystal habit	Octahedra or spherulites up to several cm in diameter
Twinning	on {111}
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	6–7
Luster	Vitreous
Streak	White
Diaphaneity	Transparent
Specific gravity	2.32–2.36
Optical properties	Uniaxial (−)
Refractive index	nω = 1.487 ; nε = 1.484
Birefringence	0.003
Pleochroism	None
Melting point	1,713 °C (3,115 °F) (β)
References

Cristobalite (/krɪˈstoʊbəˌlaɪt/) is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, SiO₂, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group, which also include coesite, tridymite and stishovite. It is named after Cerro San Cristóbal in Pachuca Municipality, Hidalgo, Mexico.

It is used in dentistry as a component of alginate impression materials as well as for making models of teeth.^[7]

^ Warr, L. N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ Deer, W. A.; Howie, R. A.; Zussman, J. (1966). An Introduction to the Rock Forming Minerals. Longman. pp. 340–355. ISBN 0-582-44210-9.
^ Mineralienatlas.
^ Cristobalite Archived 2010-07-15 at the Wayback Machine. Handbook of Mineralogy.
^ Cristobalite. Mindat.
^ "Cristobalite Mineral Data". Webmineral.
^ Anusavice, Kenneth J. (2013). Phillips' science of dental materials. Elsevier/Saunders. ISBN 978-1-4377-2418-9. OCLC 934359978.