A brief description of the nature of diorite

By: The Geology Toolkit team (Programme and web teams)

  • Duration 5 mins
  • Updated Thursday 28th September 2006
  • Introductory level
  • Posted under Geology
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Diorite is a coarse-grained intrusive igneous rock. It contains large interlocking, randomly oriented crystals. It is a dark coloured rock, usually medium to dark grey, containing many mafic crystals.

Mostly it looks like dark coloured granite.

Diorite Copyrighted image Copyright: The Open University

How was it formed?
Diorite formed deep within the Earth’s crust from cooling magma that never made it to the surface. It usually occurs as quite small intrusions often associated with larger intrusions like granite. Slow cooling produces the large crystals.

Diorite forms from magma that does not contain a lot of quartz (silica) or the light coloured minerals that make up granite.


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