PLAGIOCLASE AT EXTREME CONDITIONS: MORE THAN JUST PRESSURE EFFECTS (Invited Presentation)

Because plagioclase is a dominant mineral in differentiated planetary crusts, its behavior under extreme conditions has long been of interest to terrestrial and extraterrestrial mineralogists [1-5]. While most studies have focused on the structural response and amorphization induced by increased pressure, there is growing recognition, particularly in the meteoritic and shock community, that extreme deformation in plagioclase is dependent on many factors including pressure, temperature, strain rate [6-8].

Here we present results from some untraditional experiments suggesting that amorphization in mid-composition plagioclase (labradorite and andesine) occurs over a range of conditions such that there is an amorphization onset and an amorphization completion, which can be separated by up to 6 GPa. Furthermore, temperature affects both amorphization onset and completion where increasing T lowers amorphization onset P but increases amorphization completion.

Similarly, we show evidence of memory effects during static compression where andesine appears amorphous by 12 GPa but reverts to crystalline andesine upon decompression. However, if andesine is compressed to above ~18 GPa, the transformation is complete and it does not revert upon decompression. Both the memory effect and recognition of amorphization onset and completion points have implications for meteoritics because these studies frequently attempt to assess peak shock conditions. Because naturally shocked material is always measured on decompressed samples, however, peak shock conditions may not be obtainable.

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