EXPERIMENTAL DEFORMATION OF RHEOLOGICALLY INSIGNIFICANT MINERALS: the Al2SiO5 polymorphs

Reference

Goergen, E.T., Whitney, D.L., Zimmerman, M.E., and Hiraga, T. (2008) Deformation-induced polymorphic transformation: experimental deformation of kyanite, andalusite, and sillimanite. Tectonophysics, 454, 23-35.

The Al2SiO5 polymorphs: Kyanite, Andalusite, Sillimanite

Deformed kyanite in quartz, Menderes Massif, Turkey.(crossed polars; field of view = 4 mm)

Quartzite containing andalusite, kyanite, and sillimanite; Sivrihisar Massif, Turkey

Phase diagram of the Al2SiO5 polymorphs and the conditions of our deformation experiments. Folded kyanite, Solitude Range, British Columbia.

The Al2SiO5 polymorphs (andalusite, kyanite, and sillimanite) are common in metasedimentary rocks, and are very useful for determining pressure-temperature conditions of metamorphism.

Owing to their sluggish reaction kinetics, 2-3 polymorphs may coexist metastably, allowing interpretation of part of the P-T path if the sequence of crystallization can be determined. In rocks containing 2 or 3 polymorphs, crystallization sequence may be inferred from the relative textures of the polymorphs; e.g., comparing deformation textures. During evolving P-T conditions, the polymorphs may directly replace (pseudomorph) each other, or may grow separately in the rock matrix.

Quartz vein containing andalusite (pink), kyanite (blue), and sillimanite (difficult to see in hand sample); Hamadan, Iran (horizontal dimension of sample = 8 cm). Sepahi et al. (2004) Journal of Metamorphic Geology.

Examples of textures involving 1, 2, and 3 Al2SiO5 polymorphs:

Andalusite + sillimanite from Hamadan, Iran (Field of view = 4 mm). Sepahi, Whitney, & Baharifar (2004)

Kyanite completely replaced (pseudomorphed) by prismatic sillimanite, Thor-Odin dome, British Columbia. The former kyanite (now sillimanite) is surrounded by haloes of spinel (+ corundum + sapphirine + anorthite + cordierite) and cordierite. Goergen et al. (in review)

Kyanite + quartz, cross polarized light; Mica Creek, BC.

This might not look like an impressive rock, but in the folded area of this muscovite quartzite from Sivrihisar, Turkey, are crystals of andalusite, kyanite, and sillimanite. Whitney (2002)

Undeformed kyanite from a quartz vein, Hamadan, Iran.

Kyanite "train wreck" against garnet (illustrating that minerals can have very heterogeneous deformation depending in part on their location in a rock), Solitude Range, British Columba. Field of view ~ 2 mm.