Mica Fish

Mineral fish are elongate lozenge or lens-shaped single crystals, which are common in mylonites. They characteristically lie with their longest dimension at a small angle to the mylonitic foliation. Most common are mineral fish of large single white mica crystals known as mica fish in micaceous quarzitic mylonites. Commonly, trails of small mica fragments extend into the matrix from the tips of isolated mica fish (Fig.1).

Fig.1: Mica fish. From Jean-Pierre Burg.

Characteristic of all mineral fish is the typical elongate lozenge or lens shape, a strong preferred orientation and lack of evidence for rotation. The lozenge shape may have developed by internal deformation, erosion by recrystallisation or pressure solution, and lateral growth by precipitation of dissolved material. Of all mineral fish, white mica fish are most common. Each group (Fig.2) is considered to result from a specific combination of mechanisms:

• Groups 1 and 2 micas fish: Micas of groups 1 and 2 may have attained their apparently stable inclined position by back rotation from an original position approximately parallel to the foliation to a new stable position. The typical lens shape can be explained by removal of small recrystallized or cataclastically torn-off grains along the upper and lower parts, possibly accompanied by dissolution an/or diffusive mass transfer and by slip on (001) in a synthetic sense, that could explain the complementary rounded parts of the lens shapes. The shape of group 2 fish is thought to evolve from group 1 by drag along zones of concentrated shear, localized along the upper and lower contacts, comparable to the development of σ - type mantled clasts.
• Groups 3 micas fish: The shape of mica fish of group 3 can easily be attained by slip on (001), starting from a position parallel to the foliation.
• Groups 4 micas fish: Mica fish of group 4 are thought to have formed by antithetic slip on (001) from grains with an original high angle between internal cleavage and foliation. Alternatively they could have developed from fish of groups 1 or 2 by further removal of the upper and lower parts.
• Groups 5 micas fish: Group 5 mica fish are explained as originated from short thick micas in a similar way as group 4 ones, but with additional modification by removal along C'-type shear bands at a small synthetic angle with the foliation.
• Groups 6 micas fish: Group 6 micas may result from drag folding along C'-type shear bands.

mica_fish2020(1).png

Fig.2: a) The main types of white mica fish recognized in thin section. b) Inferred development of the different types of mica fish (after ten Grotenhuis et al. 2003). Modified from Passchier (2005).

Besides white mica, a number of other minerals can develop as mineral fish with a similar orientation with respect to the mylonitic foliation, as observed in mylonites from a variety of locations and metamorphic grade. Presently known are examples of biotite, tourmaline, K-feldspar, garnet, plagioclase, staurolite, kyanite, amphibole, diopside, apatite, rutile, hematite, prehnite, sillimanite, olivine and quartz.

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.

Photo
White mica fish in a mylonite from Sardinia. XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Sardinia. XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Sardinia. XPL image, 2x (Field of view = 7mm)
White mica fish in a mylonite from Sardinia. XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Sardinia. XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)
White mica fish in a mylonite from Sardinia. XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)
White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)
White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)
White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)
White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)	White mica fish in a mylonite from Sardinia. XPL image, 10x (Field of view = 2mm)
White mica fish in a mylonite from Sardinia. XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Sardinia. XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Sardinia. XPL image, 2x (Field of view = 7mm)
White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)
White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)
White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)
White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)
White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)
White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)	White mica fish in a mylonite from Rougbuk (Tibet). XPL image, 2x (Field of view = 7mm)