Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
Silicon dioxide
Table title:
Crystalline forms
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > Form </th><th colspan=1 rowspan=1 > Crystal symmetry Pearson symbol, group No. </th><th colspan=1 rowspan=1 > ρ g/cm3 </th><th colspan=1 rowspan=1 > Notes </th><th colspan=1 rowspan=1 > Structure </th></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > α-quartz </td><td colspan=1 rowspan=1 > rhombohedral (trigonal) hP9, P3121 No.152 </td><td class="highlighted"  colspan=1 rowspan=1 > 2.648 </td><td colspan=1 rowspan=1 > Helical chains making individual single crystals optically active; α-quartz converts to β-quartz at 846 K </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > β-quartz </td><td colspan=1 rowspan=1 > hexagonal hP18, P6222, No. 180 </td><td colspan=1 rowspan=1 > 2.533 </td><td colspan=1 rowspan=1 > 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 </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > α-tridymite </td><td colspan=1 rowspan=1 > orthorhombic oS24, C2221, No.20 </td><td colspan=1 rowspan=1 > 2.265 </td><td colspan=1 rowspan=1 > Metastable form under normal pressure </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > β-tridymite </td><td colspan=1 rowspan=1 > hexagonal hP12, P63/mmc, No. 194 </td><td colspan=1 rowspan=1 >  </td><td colspan=1 rowspan=1 > Closely related to α-tridymite; β-tridymite converts to β-cristobalite at 2010 K </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > α-cristobalite </td><td colspan=1 rowspan=1 > tetragonal tP12, P41212, No. 92 </td><td colspan=1 rowspan=1 > 2.334 </td><td colspan=1 rowspan=1 > Metastable form under normal pressure </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > β-cristobalite </td><td colspan=1 rowspan=1 > cubic cF104, Fd3m, No.227 </td><td colspan=1 rowspan=1 >  </td><td colspan=1 rowspan=1 > Closely related to α-cristobalite; melts at 1978 K </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > keatite </td><td colspan=1 rowspan=1 > tetragonal tP36, P41212, No. 92 </td><td colspan=1 rowspan=1 > 3.011 </td><td colspan=1 rowspan=1 > Si5O10, Si4O8, Si8O16 rings; synthesised from glassy silica and alkali at 600–900 K and 40–400 MPa </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > moganite </td><td colspan=1 rowspan=1 > monoclinic mS46, C2/c, No.15 </td><td colspan=1 rowspan=1 >  </td><td colspan=1 rowspan=1 > Si4O8 and Si6O12 rings </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > coesite </td><td colspan=1 rowspan=1 > monoclinic mS48, C2/c, No.15 </td><td colspan=1 rowspan=1 > 2.911 </td><td colspan=1 rowspan=1 > Si4O8 and Si8O16 rings; 900 K and 3–3.5 GPa </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > stishovite </td><td colspan=1 rowspan=1 > tetragonal tP6, P42/mnm, No.136 </td><td class="highlighted"  colspan=1 rowspan=1 > 4.287 </td><td colspan=1 rowspan=1 > One of the densest (together with seifertite) polymorphs of silica; rutile-like with 6-fold coordinated Si; 7.5–8.5 GPa </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > seifertite </td><td colspan=1 rowspan=1 > orthorhombic oP, Pbcn </td><td colspan=1 rowspan=1 > 4.294 </td><td colspan=1 rowspan=1 > One of the densest (together with stishovite) polymorphs of silica; is produced at pressures above 40 GPa. </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > melanophlogite </td><td colspan=1 rowspan=1 > cubic (cP*, P4232, No.208) or tetragonal (P42/nbc) </td><td colspan=1 rowspan=1 > 2.04 </td><td colspan=1 rowspan=1 > Si5O10, Si6O12 rings; mineral always found with hydrocarbons in interstitial spaces - a clathrasil </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > fibrous W-silica </td><td colspan=1 rowspan=1 > orthorhombic oI12, Ibam, No.72 </td><td colspan=1 rowspan=1 > 1.97 </td><td colspan=1 rowspan=1 > Like SiS2 consisting of edge sharing chains, melts at ~1700 K </td><td colspan=1 rowspan=1 > - </td></tr>
<tr> <td colspan=1 rowspan=1 > 2D silica </td><td colspan=1 rowspan=1 > hexagonal </td><td colspan=1 rowspan=1 >  </td><td colspan=1 rowspan=1 > Sheet-like bilayer structure </td><td colspan=1 rowspan=1 > - </td></tr>
</table>
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.