Schist gets its name from the Greek verb meaning to split
Heat and pressure during mountain building line up flat mica and chlorite crystals inside a rock until it peels apart into flakes thinner than a centimetre. That property, called schistosity, defines schist. It makes the rock easy to identify with a hand lens and a headache for engineers building on it.
Schist is a medium-grained metamorphic rock, most often made from fine sediment such as shale or mudstone, though volcanic tuffs and other rocks can become schist too. It usually forms during regional metamorphism of medium intensity as mountains rise. Its grains, typically 0.25 to 2 millimetres across, are easy to see under a tenfold hand lens, and more than half usually point the same way.
That alignment is the key. Platy minerals, including mica, talc, chlorite and graphite, lie in thin parallel sheets interleaved with grainier quartz or feldspar, so the rock splits readily into slabs less than 5 to 10 millimetres thick. Schistosity appears only when such flat minerals are abundant. Schists often also carry porphyroblasts, unusually large single crystals of minerals like garnet, kyanite or staurolite.
Geologists classify metamorphic rocks by texture into three groups: schist; gneiss, with weaker schistosity and thicker bands; and granofels, which has none. Schist is defined by texture alone, so its composition can vary a great deal. Until the mid-19th century, miners did not reliably distinguish slate, shale and schist.
Naming follows strict conventions. A rock is called plain schist only when its original parent, the protolith, is unknown and its minerals undetermined. If the parent was sedimentary it becomes a paraschist; if igneous, an orthoschist. When the parent is clear, schistose becomes an adjective, as in schistose metasandstone. Otherwise minerals are listed from most to least abundant, as in quartz-feldspar-biotite schist. Mica-rich schist derived from mudstone is especially common. Those same planes of weakness make schist bedrock tricky for civil engineering projects.
Source: Schist
