Experimental deformation of the Al2SiO5 polymorphs

At the University of Minnesota, we are experimentally deforming kyanite, andalusite, and sillimanite to determine their mechanical behavior, deformation mechanisms, and the effect of deformation on polymorphic transformations.

Team Al2SiO5: Eric Goergen (Ph.D. candidate), Mark Zimmerman (Senior Research Scientist & Lab Wizard), Donna Whitney (Metamorphic Rock Person)

Methods: So far, we have deformed andalusite, kyanite, and sillimanite individually and as a triplet (stacked series of discs with each polymorph in a disc) in torsion, and we have deformed a triplet in shear. All experiments are conducted in the stability field of sillimanite (see P-T diagram above), with shear strain up to 400%. The experiments are done with a gas-medium (argon) Paterson apparatus in the laboratory of David Kohlstedt at the University of Minnesota.

WHY deform the Al2SiO5 polymorphs?

• We can learn about the interaction of deformation and metamorphism by studying polymorphic transformations in experimentally deformed Al2SiO5.
• We can determine their deformation behavior (deformation mechanisms, slip systems) and therefore understand microstructures of Al2SiO5 phases in rocks.
• We can use experimental results from these pressure-temperature sensitive minerals to better understand the textures recorded by more pressure-temperature insensitive minerals (like quartz).

WHY study the Al2SiO5 polymorphs in particular?

because..

• they are P-T sensitive minerals
• they comprise a simple chemical system
• they participate in metamorphic reactions
• they are very common in Al-rich metamorphic rocks (metapelite, micaceous quartzite)
• if 2-3 polymorphs coexist, they are important for determining metamorphic paths

RESULTS so far:

Of the 3 polymorphs, sillimanite is the weakest. The experiments produced a strong crystallographic preferred orientation (CPO) in all samples, with deformation consistent with slip along planes parallel to the (001) axis (c-axis).

Petrographic view of andalusite torsion experiment. The bright bands are fine-grained sillimanite that formed during deformation.

Using a transmission electron microscope (TEM) and the help of our colleague, Dr. Take Hiraga, we have determined that the andalusite torsion experiment (photomicrographs above) produced sillimanite 'mylonites' (lighter bands) in the andalusite matrix (gray). The sillimanite occurs in subparallel bands and as fine-grained (10 nanometer sized grains) rims on andalusite.

TEM image of andalusite from a sillimanite band (mylonite) in the andalusite torsion experiment:

Figure (a) below is from an andalusite-rich region of the experiment -- note that the andalusite grains are surrounded by fine-grained sillimanite. Figure (b) shows some bent sillimanite crystals, and (c) is a close-up of some of the 'larger' (100 nm diameter) sillimanite grains.