It's kind of a tricky concept I assume, on one side you got those neat shared vertices of SiO2, on the other (water) you don't really have shared vertices, only kinda (but they still want to align themselves)
But what truly defines a glass is the existence of an alternative-form crystal that doesn't have time to form during the solidification, due to high viscosity and stuff.
Is ice the true crystal? If not it's a glass, right?
Best Answer
A crystalline substance doesn't necessarily have to be a single crystal to be deemed as such. An amorphous solid such as glass doesn't exhibit a crystalline structure even at very high levels of magnification. Glassy substances have a glass transition phase that is lower than the melting temperature. The melting point of ice formed under ordinary circumstances is ice Ih, with a melting point of 0° C. In other words, the ice we typically see is a crystalline form of ice. There are a number of other forms of ice; fortunately Kurt Vonnegut's ice nine (Cat's Cradle) is not one of them. Ice IX (as opposed to Vonnegut's ice nine) exhibits a tetragonal crystalline structure and only forms below liquid nitrogen temperatures. The ordinary ice Ih we typically encounter exhibits a hexagonal (think snowflakes) crystalline structure.
Amorphous ice can be made, but it's rather hard to prepare. Extremely pure water is needed, and the water needs to be supercooled very rapidly to liquid nitrogen temperatures. One can supercool liquid water, but any substantial disturbances or vibrations (e.g., looking at it cross-eyed) will result in that water instantaneously freezing into ordinary, everyday crystalline ice. If you quickly cool pure water far below its nominal freezing point and somehow avoid invoking any significant disturbances, you can indeed create a glassine form of ice.