Silicon dioxide
Crystalline forms

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
-

Question: What are the densities of stishovite and α-quartz?
Answer: The density of stishovite is 4.287 g/cm³ and α-quartz has a density of 2.648 g/cm³.