the glass structure

A glass is a solid that never got the chance to line up into a crystal, a frozen liquid. Imagine cooling water so fast it turns solid before the molecules can arrange into neat ice; you would get glassy water. Ordinary glass is exactly this: molten silica cooled quickly enough that its atoms are locked in the jumbled, wandering arrangement of a liquid, rather than the tidy repeating grid of a crystal like quartz.

The bonds themselves are the same strong SiO4 tetrahedra as in crystalline quartz, and each tetrahedron still shares its corners with neighbours to build a continuous 3D network. The difference is only the pattern: in quartz the linked tetrahedra repeat perfectly, while in glass they connect at slightly varied angles, so the network has short-range order (each Si still has 4 oxygens) but no long-range order (no repeating lattice). Because there is no regular lattice, glass has no single sharp melting point; instead it softens gradually over a range as it is heated, passing through the glass transition temperature where it changes from brittle solid to a thick, workable liquid.

This structure explains glass's everyday character. With no crystal planes and no grain boundaries, light passes straight through, so glass is transparent, and there are no easy slip planes, so it is hard and brittle. It also means glass is technically not at equilibrium; it is a metastable frozen liquid, though at room temperature it is frozen so solidly that the old story about medieval windows flowing over centuries is simply false. Adding other oxides (soda and lime) to pure silica lowers the working temperature, which is how cheap window glass is made.