MECHANICAL TWINNING AND SLIP IN EXPERIMENTALLY DEFORMED PLAGIOCLASES

Mechanical albite and pericline twins were induced in seven feldspars, (Na, Ca) (Al, Si)4O8, in triaxial compression tests at 800° C, 8 to 10 kb confining pressure. Two were initially in a high temperature structural state (disordered) (An1, An59). Three (An32, An39, and An53) were slightly disordered, and two (An76 and An95) had the transitional and primitive anorthite structure, respectively. No microscopic twinning was detected in comparable tests on ordered albite and oligoclase (An1, An20), feldspars in the peristerite range (An11 An14) or in a low structural state labradorite (An51). Other deformational features include lamellae 60 to 80° from the c axis and (010) slip in the opposite sense for (010) twinning. © 1969 Springer-Verlag.