| 1 | A | B | C | D | E |
| --- | --- | --- | --- | --- | --- |
| 2 | Silicon dioxide - Crystalline forms |  |  |  |  |
| 3 | Form | Crystal symmetry Pearson symbol, group No. | ρ g/cm3 | Notes | Structure |
| 4 | 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 | - |
| 5 | 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 | - |
| 6 | α-tridymite | orthorhombic oS24, C2221, No.20 | 2.265 | Metastable form under normal pressure | - |
| 7 | β-tridymite | hexagonal hP12, P63/mmc, No. 194 | - | Closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K | - |
| 8 | α-cristobalite | tetragonal tP12, P41212, No. 92 | 2.334 | Metastable form under normal pressure | - |
| 9 | β-cristobalite | cubic cF104, Fd3m, No.227 | - | Closely related to α-cristobalite; melts at 1978 K | - |
| 10 | 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 | - |
| 11 | moganite | monoclinic mS46, C2/c, No.15 | - | Si4O8 and Si6O12 rings | - |
| 12 | coesite | monoclinic mS48, C2/c, No.15 | 2.911 | Si4O8 and Si8O16 rings; 900 K and 3–3.5 GPa | - |
| 13 | 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 | - |
| 14 | seifertite | orthorhombic oP, Pbcn | 4.294 | One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa. | - |
| 15 | 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 | - |
| 16 | fibrous W-silica | orthorhombic oI12, Ibam, No.72 | 1.97 | Like SiS2 consisting of edge sharing chains, melts at ~1700 K | - |
| 17 | 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.