Pyroxene Whiskers in Cp Interplanetary Dust Particles: Evidence of Vapor Phase Growth

CP micrometeorites are highly porous and fragile interplanetary dust particles (IDP' s) that have chondritic elemental abundances. Among the mineral species found within these particles are pure enstatite crystals, which are unique among the micron-sized constituents of CP IDP' s in that they are euhedral. They exhibit at least three distinct forms, henceforth referred to as rods, ribbons and platelets, whose morphologies are totally inconsistent with those of rock-forming pyroxenes that have crystallized from a melt. Chemical and structural features of the pyroxenes were investigated by analytical scanning and transmission electron microscopy. Rods consist of single crystals that are grossly elongated along the crystallographic [ 100] direction. Ribbons are blade-shaped crystals that are morphologically similar to rock-forming pyroxenes (Fraundorf, 1981) except that they are also elongated along [ 100]. Platelets consist of very thin (200-500 A) angular crystals whose habits lie in a-c projection (i.e. they are thin parallel to [010]). Each of the three morphological forms were found to possess characteristic crystallographic defects. For example, some of the rods contain axial screw dislocations that run parallel to the lengths of the crystals. High-resolution images and diffraction data indicate that they consist exclusively of clinoenstatite containing isolated [I 00] stacking defects. Ribbons also consist exclusively of clinoenstatite that contains isolated [JOO] stacking defects. Platelets consist of pervasively intergrown ortho and clinoenstatite, together with extreme stacking disorder. The presence of whiskers and platelets in CP micrometeorites has important implications with respect to the origin of CP IDP's. Both synthetic and naturally occurring whiskers of a variety of compositions have been observed (Blakely and Jackson, 1962; Veblen and Post, 1982): apart from their distinctive morphologies, a characteristic feature of those grown by direct vapor-to-solid condensation is a rod-like morphology containing an axial screw dislocation (Blakely and Jackson, 1962). Donn and Sears ( 1963) have pointed out that nucleii present in the solar nebula are likely to have lead to whiskers and platelets containing screw dislocations. Such defects would have allowed equilibration of the saturated vapor phase with a stable crystal phase at saturations well below those required for other models of condensation. Our observations provide strong evidence that primary condensates from either the solar nebula or pre-solar environments have been preserved ii:t certain IDP's and that they can be identified and studied by electron-beam methods. Analyses of these particles should provide fundamental insights into the kinetic aspects of condensation. One intriguing aspect of this effort is the observation that growth of rods by axial screw dislocation appears to be confined to enstatite.