# Can silicon dioxide melt at 20 °C?

Can $$\ce{SiO2}$$ melt at $$\pu{20 ^\circ C}$$?

I have searched the web for $$\ce{SiO2}$$ phase diagram, but it seems to me that almost all the graph I can find have the temperature axis where the minimum temperature is in the hundreds degrees Celsius.

Is it theoretically possible that with an high enough pressure I can melt $$\ce{SiO2}$$ in a temperature range like $$\pu{-30 ^\circ C}$$ to $$\pu{50 ^\circ C}$$? Or is it something theoretically simply impossible?

• High pressure has tendency to cause solids to melt at higher, not lower temperatures. Ice is exception, as ice/water density ratio is exception as well. Generally, substances increase volume when melting and high pressure would defiinitely not to make melting easier - See Le Chatelier's principle. – Poutnik Jun 28 '20 at 15:22
• but in the same article: silica density 2.648 (α-quartz), 2.196 (amorphous) g·cm−3, molten silica density 2.08 g/cm3 at 1950 °C to 2.03 g/cm3 at 2200 °C. So there is significant melting expansion. – Poutnik Jun 28 '20 at 15:50
• ... have the temperature axis where the minimum temperature is in the hundreds of Celsius degree. I guess that means that nothing interesting happens below a few hundred degrees. :-) – Karl Jun 28 '20 at 16:00
• The diagram proposed by Alessandro Jacopson is not related to $\ce{SiO_2}$. It is the diagram of water $\ce{H_2O}$. – Maurice Jun 28 '20 at 18:35
• @Maurice Thank you, I did not add the image, see chemistry.stackexchange.com/revisions/135879/2 – Alessandro Jacopson Jun 29 '20 at 6:12

Question: Can $$\ce{SiO2}$$ melt at $$\pu{20 ^\circ C}$$?

According to experimental and calculated data values, my answer is no. See the phase diagram of pure silica based on the experimental and calculated data given in Ref.1:

Reference 1 states that:

An internally consistent data set on the thermodynamic properties of the silica polymorphs stable up to $$\pu{15 GPa}$$ (α‐quartz, β‐quartz, tridymite, cristobalite, coesite, and stishovite) and the liquid phase is presented. The data set was produced through a computer‐based assessment of the properties in which the available thermochemical (calorimetric), physical (bulk modulus and thermal expansion), and solid‐state and melting transition data (including some newly determined data on the high‐pressure polymorphs cosite and stishovite) were considered. The data set can be used to calculate phase relations at pressures of $$\pu{0.1 MPa}$$ to $$\pu{15 GPa}$$ and temperatures of $$300$$ to $$\pu{3200 K}$$. The calculated phase diagram using these data agrees quite well with the phase equilibrium determinations except for the high‐temperature part of the coesitestishovite boundary. The properties of the liquid phase obtained are also in good agreement with the available data.

Keep in mind that $$\pu{0.1 MPa} \approx \pu{1 atm}$$. According to the phase diagram, cristobalite form is in liquid form at temperature grater than $$\pu{1750 K}$$ in pressure range of $$\pu{0.1-0.7 MPa}$$. The extrapolation of that boundary would show it would never cross $$\pu{20 ^\circ C}$$ point on $$x$$-axis without going to negative pressure. Therefore, it is impossible to have any form of $$\ce{SiO2}$$ in reduced or high pressusituations at room temperature or below.

References:

1. V. Swamy Surendra, K. Saxena, Bo Sundman, J. Zhang, “A thermodynamic assessment of silica phase diagram,” Journal of Geophysical Research 1994, 99(B6), 11787-11794 (https://doi.org/10.1029/93JB02968).