B Quartz vs. Glass -- why do they have different melting points?

[Edge5]

Why quartz has a higher melting Temperature than glass?
Which one is harder and why?

 

[jedishrfu]

For one thing quartz is crystalline meaning the molecules are ordered into a structure that would need energy to break loose whereas glass is more loosely organized hence the lower temp.

Here's a more detailed discussion on it:

https://bbickmore.wordpress.com/2017/04/27/quartz-is-not-glass-so-what/

 

[johnbbahm]

A word of caution, if you are used to working with glass, quartz looks like it has cooled
off long before it actually has.

 

[snorkack]

Actually, neither of them easily has a melting temperature.

Quartz is very hard to melt because quartz is unstable above 870 Celsius, and molten silica is unstable below 1713 Celsius. In the interval between 870 and 1713 degrees, quartz tends to transform to tridymite or cristobalite, not melt. It is hard to heat quartz to melting, in the region of 1650 Celsius.

Glass does not have a melting temperature by definition because its viscosity varies continuously with temperature.

 

[cjl]

Glass does not have a melting temperature by definition because its viscosity varies continuously with temperature.

It does have a glass transition temperature though, where its properties do undergo a significant and measurable change (and this temperature is always lower than the melting point of the crystalline version of the same material).