Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Geological and Atmospheric Sciences

First Advisor

Karl E. Seifert


Oriented cores cut along axes bisecting the acute and obtuse angles between the (010) and (001) cleavage planes and normal to the a-axis were taken from single crystal low structural state sodic plagioclases (An[subscript]1 and An[subscript]18) and deformed at temperatures from 25 to 1000°C, strain rates of 10[superscript]-4 to 10[superscript]-5 s[superscript]-1, and a confining pressure of 1.0 GPa in a modified small Griggs apparatus. Water content and annealing times varied widely;The rate of disordering is faster in deformation experiments than in hydrostatic experiments but is the same in the twinning and antitwinning orientations. Transmission electron microscopy (TEM) shows that partially disordered lattices have undergone simple shear in the twinning direction. It is proposed that dislocation cores provide the increased diffusivity of the tetrahedral cations necessary for disordering;The transition from brittle-ductile to purely ductile deformation for dry samples occurs around 850°C at 1.0 GPa confining pressure. Samples deformed in the anti-twinning orientation show an overall higher strength in the brittle-ductile field than samples deformed in the twinning orientation. Dry samples run at T ≥ 900°C deform by dislocation glide with slip occurring on (010) (001), (110) (001), (010) (101), and (001)1/2 (110) systems for cores oriented in both directions. Exsolution lamellae in the oligoclase inhibit slip in unannealed samples. Deformation twinning is not observed, regardless of crystal orientation, for T ≥ 900°C;True deformation twinning occurs when T ≥ 800°C in disordered crystals favorably oriented for albite and pericline twinning. Dislocations were not observed associated with these twins. Disordered samples twinned and were weaker than ordered samples deformed under similar conditions. No such effect was seen in samples deformed in the anti-twinning orientation. The critical resolved shear stress for both albite and pericline deformation twinning is approximately 200 MPa;Water significantly lowers the onset of plasticity and sample strength. Samples deformed in talc assemblies at T ≥ 800°C show rapid disordering and a high degree of cataclasis (due to a high P[subscript] H2O) with evidence of recrystallization.



Digital Repository @ Iowa State University,

Copyright Owner

Matthew Joseph Kramer



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144 pages