On the nature of Guinier-Preston zones in meteoritic and lunar orthopyroxene

Introduction: Upon cooling in their host rocks, orthopyroxenes undergo cation ordering and exsolution processes. The state of order achieved during cooling [1] and the type and morphology of the exsolution products depend on the rate of cooling. Often, the formation of stable exsolution phases is preceded by the formation of metastable phases, e.g. Guinier-Preston (GP) zones. GP zones consist of disc-shaped platelets which are coherently intergrown with the orthopyroxene host, oriented parallel to (100), only one to a few unit cells wide. They are enriched in Ca but differ structurally from monoclinic clinopyroxene [2-5]. In a previous model [3], GP zones being several unit cells wide were interpreted to possess Pbc21 symmetry and to consist of two half-unit cell layers with Wo0 and Wo50 compositions. This model assumes that the Ca cations have completely ordered in one half-unit layer of 9 Å width. Methods and motivation: We have readdressed the question of the structure of GP zones, using transmission electron microscopy (TEM). The TEM observations were performed on orthopyroxene from the Johnstown achondrite and a lunar troctolite (76535) [3, 4]. Energy-dispersive X-ray analyses recorded on the TEM confirm the enhanced Ca content within the zones. However, a quantitative determination of the Ca content across the GP zones is presently almost impossible due to their small size. The Johnstown orthopyroxene contains GP zones apparently one single unit cell in width (18 Å), whereas the lunar orthopyroxene is penetrated by GP zones up to three unit cells in width. To better constrain the structure of the GP zones we used selected area electron diffraction and highresolution TEM combined with numerical simulation. To determine the space group symmetry of GP zones and orthopyroxene hosts, the electron diffraction patterns have been taken along the crystallographic a, b and c axes. Results and discussion: Electron diffraction. The diffraction patterns of Johnstown and lunar host orthopyroxenes show absences of reflections that are characteristic of the space group Pbca. This is the space group expected for orthopyroxene. However, the electron diffraction patterns taken from host regions containing GP zones reveal a reduction in symmetry. In case of the Johnstown orthopyroxene, which contains GP zones of only one single unit cell in width, the (hk0) electron diffraction pattern taken along the c-axis shows streaks along the a*-axis (inset in Fig. 1), possibly indicating violation of the aglide parallel to (001). In the (0kl) diffraction pattern of the GP zones, the even and odd 0k0 reflections are both present. The preservation of odd reflections cannot be explained by double diffraction and thus indicates violation of the b-glide parallel to (100). The (h0l) diffraction pattern shows the absence of odd 00l reflections, which is due to c-glide on the (010) plane. Except for (hk0), similar diffraction patterns are obtained from GP zones in the lunar orthopyroxene. Here, elongated odd h00 reflections in the (hk0) pattern demonstrate clear violation of the a-glide (inset in Fig. 2). This observation confirms previous TEM results, obtained on the same lunar sample [3]. Altogether, the diffraction data point to a space group of GP zones that has a lower symmetry than the previously suggested space group Pbc21[3]. The violation of aand b-glide is compatible with the monoclinic space group P21/c that has been proposed by [6].