kaolinite

Kaolinite is the pure white clay mineral at the heart of porcelain, the china clay that gives fine tableware its whiteness and translucency. Its formula is Al2Si2O5(OH)4, and its structure is the simplest of all the clays: a single tetrahedral silica sheet bonded face to face with a single octahedral aluminium-hydroxide sheet, making a two-layer sandwich called a 1:1 or TO layer. These TO layers then stack on top of one another like sheets of paper, held together by hydrogen bonds between the hydroxyl surface of one layer and the oxygen surface of the next.

That hydrogen bonding is strong enough to lock the interlayer at a fixed spacing of about 0.72 nm, so, crucially, kaolinite does not swell, water cannot force its way between the TO layers the way it does in a smectite. This makes kaolinite only moderately plastic compared with the swelling clays, but clean, white, and stable, exactly what porcelain wants. Its ideal composition is close to 39.5 percent Al2O3, 46.5 percent SiO2, and 14 percent chemically bound water, water that is driven off around 500 to 600 degrees C when the layers collapse to a disordered metakaolin.

On firing hotter still, metakaolin transforms near 980 degrees C and then reacts on toward the target of a porcelain firing: fine needle-like crystals of mullite (an aluminium silicate) growing in a silica-rich glass, the microstructure that gives fired porcelain its strength and translucency. Kaolinite is a weathering product of feldspar, so nature quietly turns yesterday's flux into tomorrow's clay, and the ceramist reunites them in the kiln.