HIGH-TEMPERATURE DEFORMATION OF DIOPSIDE .4. - PREDOMINANCE OF (110) GLIDE ABOVE 1000-DEGREES-C

Gem quality single crystals of diopside were deformed in orientations labelled [3] and [4] selected to promote either (100)[010] and/or (010 )[100], and {110}[001], respectively. Transmission electron microscopy (TEM) investigations performed on samples deformed in orientation [3] show that (100)[010] was activated and [010] dislocations are in clim b configurations. A number of 1/2[110] dislocations are also detected although the Schmid factors of {110}1/2[110] glide systems were very l ow. These dislocations also lie in climb configurations. The selected thermodynamic conditions, especially the partial pressure of oxygen, a llowed a large amount of partial melting to occur leading to non-intri nsic creep data. In samples deformed in orientation [4], {110}[001] gl ide was activated with a limited amount of partial melting. TEM invest igations show that most of the dislocations are straight, and of screw character parallel to [001]. A few 1/2[110] dislocations in climb con figurations are also detected. For an applied stress sigma = 147 MPa a nd at P(O)2 almost-equal-to 2.7 X 10(-16) MPa the creep law for {110}[ 001] glide is ln epsilon = 27.33 - 518/RT (epsilon in s-1, R = 8.32 kJ mol-1 K-1). Comparison of all the data collected so far on high tempe rature creep of diopside indicate that the easiest glide systems above 1000-degrees-C are {110}1/2[110]. They are followed in activity by {1 10}[001], then by (100)[001]. Other potential systems appear to be app reciably stronger and should remain marginal in natural deformation of diopside and similar C2/c clinopyroxenes. Climb of 1/2[110] dislocati ons might become relevant at temperature above 1200-degrees-C.