Silicon dioxide

Silica is probably the most versatile inorganic compound in human civilisation. It makes up ~12 % by mass of the Earth's crust, and on its own it has engendered three major industries: glass (since -1500), structural ceramics (since antiquity), and silicon electronics (since 1947). Its structure rests on SiO₄ tetrahedra sharing corners — a geometry whose arrangement determines the polymorph. α-quartz is the most stable up to 573 °C, followed by β-quartz, cristobalite, tridymite, and finally the vitreous melt with no long-range crystallinity.

The glass industry exploits this latter property: cooled rapidly, SiO₂ trapped in a disordered state is an amorphous solid transparent in the visible, hard and chemically inert. Adding Na₂CO₃ (lowers melting point from 1700 to 1500 °C) and CaO (hardens glass, makes it insoluble) gives soda-lime glass — that of bottles and windows, ~95 % of world production (~150 Mt/yr). Borosilicate (Pyrex) incorporates B₂O₃ to resist thermal shock. Optical fibres and wafer substrates use ultra-pure SiO₂ (>99.9999 %).

The biological side is subtler: respirable crystalline silica causes silicosis, an irreversible pulmonary fibrosis that killed mass numbers of miners and quarry workers in the 19th century ("stone cutters" died on average at 40). The WHO classifies crystalline silica as a Group 1 carcinogen — while its amorphous form (fumed silica, diatomaceous earth) is largely harmless. This structural distinction between crystalline and amorphous for the same chemical compound is one of the most instructive cases in mineral toxicology.