All minerals, exceptición a small percentage are presented as random aggregates of mineral grains in rocks of the crust. 
These grains are, in general, the outer faces, but, being crystalline, possesses a domestic order which becomes evident in the peel, optical and x-ray diffraction
House of Music Oporto, Koolhaas
Under certain conditions two or more crystals grow together in a rational and symmetrical. Crystallographically controlled growth such sets are called twins.
surface whereby the two individual crystals are united in twin is known as surface composition of the twin.
twinned crystals are designated usually called twinning 
as contact or penetration twins.
as contact or penetration twins. contact
The twins have a defined binding surface that separates the cristales.Las penetration twins are formed by different interpenetrating crystals with irregular joint surface.
Twinned Pyrite
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