Deformation of pyroxenes

The pyroxenes are the most abundant minerals, after olivine, in perido-tites, which are the dominant constituents of the upper mantle. It is not surprising, therefore, that there has been considerable interest in the mechanical properties of the pyroxenes, and a review has recently been given by Doukhan et al. (1986). The orthorhombic pyroxenes deform by slip and by a shear transformation that produces monoclinic lamellae (one or a few unit cells thick) parallel to (100). Coe and Kirby (1975) and McLaren and Etheridge (1976) have shown that the shear transformation is achieved by the glide of partial dislocations of b = 0.83[001] in (100), which leave partial dislocations of b = 0.17[001] terminating the shear lamellae. The dominant slip system is (100) [001]. Recent TEM observations by van Duysen, Doukhan, and Doukhan (1985) suggest that the dislocations associated with this slip system may be dissociated into four partials and that the slip system (100) [010] may also be activated. These observations are discussed in Section 9.9.1. 

The deformation microstructures of monoclinic pyroxenes are considered in Section 9.9.2. Optical microscope observations (Griggs, Turner, and Heard 1960 Raleigh 1965) indicate that the dominant slip system in monoclinic pyroxenes is also (100)[100]. However, van Duysen and Doukhan (1984) found by TEM that in naturally deformed a-spodumene the activated slip systems are (110) [001] and [lT0) 110 . In specimens of a-spodumene deformed by scratching, they also observed interesting microstructures of dislocations and faults that may be related to the twins observed in deformed diopside by Kirby and Christie (1977). 

It is clear that all fault vectors must be of the form R = [Ouw], and the proposed model (whose details need not concern us here) indicated that 

2 Dislocations and faults in deformed monoclinic a-spodumene and diopside. Van Duysen and Doukhan (1984) examined the dislocations in both naturally and experimentally deformed single crystals of a-spodumene. The naturally deformed specimens were characterized by a heterogeneous distribution of dislocations (p 10 cm ). The dislocations are not dissociated and most have b = [001] and lie in the crystallo-graphically equivalent planes (110) and (iTO). Other dislocations, probably with b = [110] and [TlO], were also observed and commonly interact to form irregular networks according to the reaction 

From the fringe contrast, they estimated w to be in the range 0.1 iv 0.25. 

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