Deformation and Metamorphism

TEXTURES OF METAMORPHIC ROCKS
The texture of igneous rocks is described in terms of grain/crystal size and
arrangement.
For sedimentary rocks, the term “texture” covers three properties: grain size,
grain shape and sorting of grains.
However, when describing the texture of metamorphic rocks, the alignment
of grains is of particular importance. Metamorphic rocks are either nonaligned
or foliated.
Non-aligned Rocks
The grains of non-aligned rocks show no “preferred orientation”, or layering.
Their appearance is the same in all directions (see the pictures of quartzite,
marble and hornfels earlier these notes).
Non-aligned rocks are formed either:
or
i. when there is no directed pressure acting on the rock.
(e.g. hornfels is formed by thermal metamorphism)
ii. when the rock contains no platy (flat) minerals.
(e.g. sandstone → quartzite and limestone → marble)
These transformations may occur in both thermal and regional
metamorphism because the mineral grains in both quartzite and marble are
not flat and therefore cannot align in any particular direction. This is shown
in the diagram below.
The adjacent diagram shows a rock (e.g.
quartzite) which has a non-aligned texture,
i.e. it is not foliated. Even directed pressure
associated with regional metamorphism
cannot produce a foliated texture in this
rock because the grains are rounded rather
than platy.
Topics 1.3.3 & 1.3.4 Deformation & Metamorphism Page 16 of 33