This rock comes from an area which has undergone similar metamorphism to that which produced the granitic gneiss specimen, but the original composition of the rock was quite different, so that re-crystallisation has produced a rock composed virtually entirely of biotite.

Most of the flat, platy crystals are lined up into layers, and the rock splits easily along them.

What makes the crystals line up like this?  Press your hands together, to represent the pressure on the buried rocks.  Now try an imagine a crystal growing between them.  It is easier for a crystal to grow sideways than to grow against the pressure.

Biotite is a mineral in the mica family, which includes the white mica, muscovite, which is also found in schist.  The crystals are flat or flaky because of the way the atoms are arranged to make up the crystal.  They are in sheets, with planes of weakness between them, so that the crystal breaks easily in that direction.

Fluids from a nearby pegmatite soaked in and carried the chemical elements necessary to make the green variety of beryl, we call emerald.  This rock in the picture had a small emerald in it.  This rock was found near Cue in Western Australia.

Text and photographs from;
Geological Museum Perth

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