Three-dimensional microstructure of symplectite minerals in the Horoman peridotite. A preliminary analysis.

Abstract

Pyroxene-spinel symplectites of possibly garnet origin occur characteristically in the Horoman peridotite. The three-dimensional microtexture of these symplectic intergrowths from a fertile spinel lherzolite is described quantitatively integrating successive two-dimensional BSE images. Symplectite structure is characterized by coarse-grained branch structure of clinopyroxene grains and spinel grains having rod, planar, cylinder, tubular and branch structure within orthopyroxene crystals. Symplectite clinopyroxene grains having branch structure are connected to clinopyroxenes in a fine-grained mineral aggregate around the symplectite. This suggests that symplectite pyroxenes possibly grew from the outer side inwards sharing the crystal axis with pyroxenes around the symplectite. Modal variations of clinopyroxene remain almost constant in one symplectite (average 23%). While the width of the spinel grain in two dimensions is almost the same as in the symplectite, it can be slightly changed by the angle between the cut plane and the elongation of spinel structures. Mineral distributions of spinel and clinopyroxene are closely related to each other. Spinel occurring at the grain boundaries between clinopyroxene and orthopyroxene has planer shape. Almost all spinel grains are located at the grain boundaries between the orthopyroxene and clinopyroxene in three dimensions. These suggest that spinel grains preferentially nucleated on grain boundaries between orthopyroxene and clinopyroxene after the formation of pyroxene domains as suggested by Obata et al. (1997).