Silicon dioxide
Crystalline forms
SiO₂, more so than almost any material, exists in many crystalline forms. These forms are called polymorphs.

Silicon dioxide

Section Title: Crystalline forms
Table Section Text: SiO₂, more so than almost any material, exists in many crystalline forms. These forms are called polymorphs.

Form	Crystal symmetry Pearson symbol, group No.	ρ g/cm3	Notes	Structure

α-quartz	rhombohedral (trigonal) hP9, P3121 No.152	2.648	Helical chains making individual single crystals optically active; α-quartz converts to β-quartz at 846 K	-

β-quartz	hexagonal hP18, P6222, No. 180	2.533	Closely related to α-quartz (with an Si-O-Si angle of 155°) and optically active; β-quartz converts to β-tridymite at 1140 K	-

α-tridymite	orthorhombic oS24, C2221, No.20	2.265	Metastable form under normal pressure	-

β-tridymite	hexagonal hP12, P63/mmc, No. 194	Closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K	-

α-cristobalite	tetragonal tP12, P41212, No. 92	2.334	Metastable form under normal pressure	-

β-cristobalite	cubic cF104, Fd3m, No.227	Closely related to α-cristobalite; melts at 1978 K	-

keatite	tetragonal tP36, P41212, No. 92	3.011	Si5O10, Si4O8, Si8O16 rings; synthesised from glassy silica and alkali at 600–900 K and 40–400 MPa	-

moganite	monoclinic mS46, C2/c, No.15	Si4O8 and Si6O12 rings	-

coesite	monoclinic mS48, C2/c, No.15	2.911	Si4O8 and Si8O16 rings; 900 K and 3–3.5 GPa	-

stishovite	tetragonal tP6, P42/mnm, No.136	4.287	One of the densest (together with seifertite) polymorphs of silica; rutile-like with 6-fold coordinated Si; 7.5–8.5 GPa	-

seifertite	orthorhombic oP, Pbcn	4.294	One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa.	-

melanophlogite	cubic (cP*, P4232, No.208) or tetragonal (P42/nbc)	2.04	Si5O10, Si6O12 rings; mineral always found with hydrocarbons in interstitial spaces - a clathrasil	-

fibrous W-silica	orthorhombic oI12, Ibam, No.72	1.97	Like SiS2 consisting of edge sharing chains, melts at ~1700 K	-

2D silica	hexagonal	Sheet-like bilayer structure	-

Sentence(s)

The density of stishovite is 4.287 g/cm³ and α-quartz has a density of 2.648 g/cm³.

final_sentence: The density of stishovite is 4.287 g/cm³ and α-quartz has a density of 2.648 g/cm³.