Penetration twin

A penetration twin is a type of twinned crystal.  This is where crystals are joined either as contact twins, where the crystals share one surface, or as a penetration twin,where the crystals grow across each other (as in the crystal shown above).

Twinning is a feature of several minerals and can be a characteristic feature of different types of feldspar (see below).  The crystal above is a mineral that you should know; can you identify it?