a network former
In an oxide glass there is a division of labour, and the network formers are the ones who build the scaffolding. A network former is an oxide whose cation is small and highly charged, so it grabs just three or four oxygens into a tight little polyhedron and then links those polyhedra corner to corner into a continuous, springy three-dimensional web. That web is the glass. The classic formers are silica SiO2 (silicon in oxygen tetrahedra), boric oxide B2O3 (boron in oxygen triangles), and phosphorus pentoxide P2O5. Germania GeO2 and arsenic oxide join the club too.
What qualifies a cation as a former is exactly the Zachariasen recipe. The cation needs a low coordination number, three or four, and a bond to oxygen strong enough and directional enough that the polyhedra prefer to share only corners. Silicon fits perfectly: the Si-O bond is roughly half covalent, the SiO4 tetrahedron is rigid, and tetrahedra bolt together at flexible oxygen hinges. The strength of that Si-O bond is why silica is such a reluctant crystallizer and such a superb glass-former, and also why silica-based glasses are so refractory and chemically durable.
It is worth naming the whole family. Beyond the true formers sit the intermediates, oxides such as Al2O3 and TiO2 that cannot build a network alone but can slot into an existing one and take a former-like role, patching broken bonds. And opposite the formers stand the network-modifiers, oxides like Na2O and CaO that break the network rather than build it. Almost every practical glass is a silica network, sometimes reinforced by a second former like B2O3, loosened by modifiers to make it meltable.
Boric oxide earns its keep as a second former in Pyrex. Add B2O3 to a silica network and the boron slots in as its own set of oxygen triangles, tightening the web and dropping the thermal expansion. That is why borosilicate labware shrugs off the thermal shock of boiling water in a cold beaker, while plain soda-lime glass, whose network is heavily broken by modifiers, would crack.
Silica and boric oxide are both network formers; together they build the low-expansion skeleton of borosilicate glass.
Being a former is about the cation's size, charge, and bonding, not just its identity. Alumina is a chameleon: an intermediate that behaves like a former when it can enter tetrahedral sites but like a modifier otherwise.