mica
/ MY-kuh /
Mica is the sheet silicate that splits into thin, springy, transparent flakes, the shiny flecks in granite and the see-through window you once found in old stove doors. It belongs to the 2:1 layer silicates: each structural layer is an octahedral sheet sandwiched between two tetrahedral silica sheets, a TOT sandwich. The common micas are muscovite (a pale, aluminium mica), biotite (a dark, iron-magnesium mica), and phlogopite (a magnesium mica prized for heat resistance).
What sets mica apart from a clay is the interlayer. In mica about a quarter of the tetrahedral silicon is replaced by aluminium, which leaves each TOT layer with a substantial negative charge; that charge is balanced by potassium ions locked tightly between the layers, keying one sandwich to the next. The potassium bonds are strong enough that mica does not swell with water and does not develop clay-like plasticity, yet weak compared with the bonds inside the TOT sandwich, so mica cleaves with astonishing perfection into flat, flexible basal sheets you can peel apart almost endlessly.
That perfect cleavage plus mica's electrical insulation, heat resistance, and transparency made sheet mica a classic dielectric and window material in electrical and high-temperature gear, and ground mica is a filler in paints, plastics, and some ceramics. For the ceramist, the more important cousin is illite, a fine-grained, mica-like clay that is the commonest clay mineral in ordinary earthenware bodies; it behaves as a mildly plastic clay because its layers are smaller, more disordered, and less tightly keyed than true mica.
Split a muscovite crystal with a knife tip and it peels into transparent leaves thinner than paper, all along the same basal plane. You are separating TOT sandwiches held only by their interlayer potassium, while the strong bonds inside each sandwich never break.
Locked by interlayer potassium: mica cleaves perfectly but, unlike clay, does not swell or turn plastic.
The interlayer cation is the whole difference. Swap mica's tightly bound potassium for loosely held, hydratable cations and you get a swelling smectite; the same 2:1 sandwich behaves completely differently.