Pyroxenes deformation

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). 

Cataclastically deformed plagioclase could be formed near the surface of the Moon and asteroids during heavy bombardment by meteoroids. An example of fractured plagioclase is observed in the eucrite Yamato (Y)-792510 (Fig. 7.3) consisting mainly of plagioclase and pyroxene which show undulatory extinction. Compared with pyroxene, plagioclase grains are heavily fractured. 

The bands are usually alternatingly light (quartz and feldspars) and dark (layer silicates, amphiboles, pyroxenes, garnet, etc.), and often display preferred orientation of mineral grains. Because metamorphic rocks display abundant field and petrographic evidence of deformation, metamorphic banding has always been attributed to stress or to the strain produced by it. 

Enstatite-hypersthene, (Mg,Fe)Si206, the second most abundant mineral in upper mantle rocks, is orthorhombic (Pbca). Pyroxenes are chain silicates, with SiOs chains extending along the c-axis. Dislocations with [001] Burgers vectors dominate deformation with ghde on the (100) and (010) planes [107-112]. There are also (100)[010] dislocations with a [100] line direction [311,312] [Fig. 13(a)]. The critical resolved shear stress (CRSS) for the (100) [001] slip system is significantly smaller than for (010)[001]. With a limited number of slip systems, it is inferred that climb... 

Mechanical (101) [101] twins have been identified in experimentally deformed hornblende single crystals, as well as dislocations on the (100)[001] slip system [333,334]. In hornblendes from naturally deformed rocks dislocations on (hkO) planes were documented, mainly [001] screws [335-338]. A systematic investigation of dynamically recrystallized hornblende from a high-temperature shear zone discovered microstructures typical of dislocation creep, with subgrain boundaries and free dislocations [313]. The primary slip system is (100)[001] consistent with experimental results. Secondary, slip systems are (010)[100] and 110)5<110>. There is evidence for cross-slip of [0 01] screws producing heUcal microstructures [Fig. 13(b)]. Amphibole structures are intermediate between pyroxenes and sheet silicates and indeed chain multiplicity faults have been described [339] and transitional structures may be facilitated by movement of partial dislocations [340]. 

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