Deformation
In minerals
As deformation progresses, a mineral can display the following textures (e.g., quartz) - this correspond to brittle to brittle-ductile deformation that tends toward grain-size reduction:
Undulose extinction (especially in quartz);
Development of deformation bands or lamellae;
The bands and lamellae rotate;
Kinkage, kink band – rotated band or lamellae, best detected in mineral with twining (e.g., plagioclase).
Sub-grains (polygonal) develop;
Sub-grains rotate;
The rotated sub-grains recrystallize, producing a rock with an homogenized grain size (equigranular).
Saccharoidal texture – granular texture, for quartz- and/or carbonate-rich rock that can be weathered (i.e., grains come out easily, re-enforcing the ‘sugar pile’ look of the rock).
For grain boundaries - as deformation progresses and as temperature increases:
Buldging recrystallisation – occurs at low/moderate temperature and corresponds to moderate brittle deformation. This produces irregular grain boundaries. As deformation progresses, grain boundaries can develop:
Sub-grain develop and rotate (and can recrystallize) along grain boundary – produces porphyroclasts rimed by small polygonal grains;
Mosaic texture – grain boundary recrystallizes at a high temperature and this reduced the number of grains compared to the previous stage (as the small sub-grains developed earlier rejoin with the nearest porphyroclast), and produces highly irregular and interlobate grain boundaries.
Not to be mistaken for deformation:
Fractures in pentlandite (due to thermal stress) – the volume of pentlandite decreases during cooling, and this decrease is sufficient to produce fractures and holes in the pentlandite.
Desiccation figures – occurs in drying mud pool. Can also occur in extensively altered rocks, when hydrothermal alteration induces mass (and volume) loss (e.g., carbonatized serpentinite).
Metamorphic context
Common processes in a metamorphic context:
Recrystallisation under a dissolution-recrystallisation process – can form layering/banding texture in the rock
Solid-state diffusion (changing the shape and/or type of grains)
Fluage (e.g., galena) – more resistant minerals (e.g., pyrite) can be broken-up as softer mineral are being deformed.
Textures:
Schist – foliated phyllosilicate-rich rock.
Gneiss – foliated phyllosilicate-poor rock.
Lepidoblastic texture – platy or tabular minerals align to produce a planar fabric.
Nematoblastic – prismatic mineral (e.g., amphibole) that are isolated in a rock are oriented to produce a linear fabric.
Oblique foliation (common in mylonite) – elongated grains defining a foliation that is oblique to the main foliation.
Ribbon – strongly elongated quartz (mechanical stretching or dissolution-recrystallisation process), common in mylonite.
Augen (eyed gneiss) – porphyroclast with similar orientation in a finer grained matrix.
Boudinage – rock that contain layers with distinct competency, with the least and most competent rocks responding to deformation by ductile (fluage) and brittle deformation, respectively.
Durchbewegun – intense deformation induce the fluage of all the minerals.
Schlieren – elongated minerals (or small recrystallized grain) define planar structures in part of the rock (not penetrative).
Strain fringes (ombre de pression) – mineral dissolved under pressure (e.g., quartz) and recrystallized in low-pressure area, along the edge of a rigid mineral or clast (strain fringes can be syntaxial, i.e., growing from the core mineral, or antitaxial, or complex).
Mica fish – sigmoidal shaped mica that is common in mylonite, see an example on the alexstrekeisen website.