olivine

Olivine is the olive-green mineral that makes up most of the Earth's upper mantle, the deep rock we almost never see. For a ceramist it is the textbook nesosilicate: isolated SiO4 tetrahedra with no corner-sharing at all, cemented together entirely by magnesium and iron ions sitting in the octahedral holes between them. Its general formula is (Mg,Fe)2SiO4, and it runs as a continuous solid solution between two end-members, magnesium-rich forsterite (Mg2SiO4) and iron-rich fayalite (Fe2SiO4).

Because the tetrahedra never link to one another, olivine's strength comes from the dense, strongly bonded cation-oxygen network wrapping every island. Each Mg2+ or Fe2+ sits in six-fold (octahedral) coordination with corner oxygens of several tetrahedra, and this three-dimensional stitching makes olivine hard (about 6.5 to 7 on the Mohs scale) and, at the magnesium end, remarkably refractory. Pure forsterite melts at roughly 1890 degrees C, whereas swapping magnesium for iron drops the melting point steeply toward fayalite's roughly 1200 degrees C, a reminder that the cation, not the silicon, controls the meltability of an island silicate.

In ceramics, magnesium-rich olivine and its firing product are used to make forsterite refractories, basic (magnesia-family) bricks and shapes that resist iron-rich, basic slags in steelmaking, and forsterite ceramics prized for low dielectric loss. Naturally occurring olivine sand is also a foundry moulding material. The lesson for a beginner: the least-connected silicate is not the weakest; olivine shows that a well-bonded cation network can make an island silicate genuinely high-melting and hard-wearing.