Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Strain fringes

Rigid objects in a ductilely deforming rock cause local perturbation of the stress field and flow pattern. In the case of low temperature deformation and high fluid pressure, increased pressure solution may occur adjacent to the rigid object on the side of the shortening ISA (Instantaneous Stretching Axes), while extensional gashes may open on the contact of the object and the matrix on the side of the extensional ISA. New crystalline material may grow in these gashes and form strain shadows on both sides of the rigid core object. If the crystals in such strain shadows are elongate or fibrous, these are known as strain fringes. The combination of core object and fringes is called a fringe structure.

Fringe structures can be syntaxial, antitaxial or complex (Fig.1). Notice that in fringe nomenclature, syntaxial fringes have the growth surface between the fringe and the wall rock, and antitaxial fringes between the fringe and the core object. This can be confusing since it may seem to be contrary to the terminology used for veins.

Fig.1: Syntaxial, antitaxial and complex fringe structures.

Syntaxial fringe: fringe with the growth surface on the outside of the fringe (between fringe and wall rock). The fibres are of the same mineral type as the rigid object and grow with crystallographic continuity on the object wall, hence the term syntaxial. Syntaxial fringes have been observed around crinoid stem fragments as core object in limestones (crinoid-type fringes; Ramsay and Huber 1983) but these are relatively rare.

Antitaxial fringe: fringe with the growth surface between the rigid object and the fringe. Possible continuity with the wall rock but no continuity with the object, hence the name antitaxial. Antitaxial fringes are the most common fringes type, probably because the core object is usually a mineral with contrasting properties from the matrix, such as pyrite in a pelitic, carbonatic or quartz-feldspar rock.

Complex fringe: fringe with antitaxial and syntaxial composite growth. Complex fringes around pyrite crystals, with calcite and a quartz rim, have been reported.

Strain fringes are normally further classified according to their growth direction (Fig.2) ranging from displacement-controlled growth which means that fibres follow the opening path of the fringe to face controlled where fibres grow normal to faces of the core object.

Face-controlled: fibres grow normal to the core-object face.
Displacement-controlled: fibres grow parallel to the relative displacement of fringes and core-object during progressive deformation.

The geometry of the fringes depends on many features such as the shape of the core object, the flow regime in the surrounding matrix, and whether fibres growth is displacement or face controlled. Depending on the P-T conditions, the fibres or strain fringes may be deformable (high temperature conditions) or rigid (lower temperature conditions).

Fig.2: Face- and displacement-controlled fringe structures.

Fringes on spherical core objects: The simplest type of fringe develops on spherical core objects. A common example for spherical core objects are globular aggregates of pyrite with a raspberry-like external form known as framboidal pyrite.

In coaxial pure shear, displacement-controlled fibres are straight, whereas in face-controlled deformation fibres show inward curvature of fibres that gradually decrease in width. The development of fibres in simple shear progressive deformation is more complex. The strain fringe will rotate as a whole while it is pulled away from the rigid core object. The fringes at the core object surface continue to grow and so a curved geometry is caused (Fig.3).

fringes2020(1).png

Fig.3: Fringes on spherical core objects during pure and simple shear deformation.

Fringes on angular core objects: Most fringes in nature develop on angular core objects with planar faces such as pyrite or magnetite crystals in fine-grained metapelite or carbonaceous slate at low metamorphic grade. The difference to fringes on spherical core objects is the contact suture that can be built between the fibres growing on different crystal faces. Such sutures are attached to the corner between two faces of the core object (Fig.4).

fringes2020(2).png

Fig.4: Fringes on angular core objects during pure and simple shear deformation.

Furthermore, fringes can be rigid, i.e. fringes are more competent than the matrix, or they are deformable, i.e. matrix deforms the strain fringes. As a result of interaction of fringe growth, rotation and deformation several geometries of fringes are possible.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Pyrite with quartz strain fringes. PPL image, 2x (Field of view = 7mm)	Pyrite with quartz strain fringes. XPL image, 2x (Field of view = 7mm)	Pyrite with quartz strain fringes. XPL (with Compensator plate) image, 2x (Field of view = 7mm)
Pyrite with quartz strain fringes. XPL image, 2x (Field of view = 7mm)	Pyrite with quartz strain fringes. XPL image, 2x (Field of view = 7mm)	Pyrite with quartz strain fringes. PPL image, 2x (Field of view = 7mm)
Pyrite with quartz strain fringes. XPL image, 2x (Field of view = 7mm)	Pyrite with quartz strain fringes. XPL (with Compensator plate) image, 2x (Field of view = 7mm)	Pyrite with quartz strain fringes. PPL image, 2x (Field of view = 7mm)
Pyrite with quartz strain fringes. XPL image, 2x (Field of view = 7mm)	Pyrite with quartz strain fringes. XPL (with Compensator plate) image, 2x (Field of view = 7mm)	Pyrite with quartz strain fringes. PPL image, 1,5x (Field of view = 9mm)
Pyrite with quartz strain fringes. XPL image, 1,5x (Field of view = 9mm)	Pyrite with quartz strain fringes. XPL (with Compensator plate) image, 1,5x (Field of view = 9mm)	Pyrite with quartz strain fringes. XPL image, 1,5x (Field of view = 9mm)
Pyrite with quartz strain fringes. XPL image, 10x (Field of view = 2mm)	Pyrite with quartz strain fringes. XPL image, 10x (Field of view = 2mm)	Pyrite with quartz strain fringes. XPL image, 10x (Field of view = 2mm)
Pyrite with quartz strain fringes. XPL image, 10x (Field of view = 2mm)	Pyrite with quartz strain fringes. XPL (with Compensator plate) image, 10x (Field of view = 2mm)	Pyrite with talc strain fringes. XPL image, 20x (Field of view = 1mm)
Pyrite with talc strain fringes. PPL image, 20x (Field of view = 1mm)	Pyrite with talc strain fringes. XPL image, 20x (Field of view = 1mm)	Pyrite with talc strain fringes. PPL image, 10x (Field of view = 2mm)
Pyrite with talc strain fringes. XPL image, 10x (Field of view = 2mm)	Pyrite with talc strain fringes. PPL image, 20x (Field of view = 1mm)	Pyrite with talc strain fringes. XPL image, 20x (Field of view = 1mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. PPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL (with Compensator plate) image, 1x (Field of view = 9mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. PPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL (with Compensator plate) image, 1x (Field of view = 9mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. PPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. PPL image, 1x (Field of view = 9mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 1x (Field of view = 9mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. PPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL (with Compensator plate) image, 2x (Field of view = 7mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. PPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL (with Compensator plate) image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. PPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL (with Compensator plate) image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. PPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 10x (Field of view = 2mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 10x (Field of view = 2mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL (with Compensator plate) image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)
Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL image, 2x (Field of view = 7mm)	Pyrite crystals (black) with quartz-calcite strain fringes. "Pizarras de luarca formation" slate, casio, Spain. XPL (with Compensator plate) image, 2x (Field of view = 7mm)