Fig. 4.

Schematic representation of a “defect” created by the replacement of oxygen by carbon in the lattice of an ortho-enstatite (MgSiO₃) structure. The Si, O, Mg, and C atoms are shown in yellow, red, green, and dark gray, respectively. The chains of Q² SiO₄ tetrahedra in the structure are oriented along the crystallographic c axis indicated by the arrow. Two BO atoms connecting two pairs of SiO₄ tetrahedra in the neighboring chains have been substituted by the carbon atom that now bonds to four Si atoms (shown in pink) and cross-links the two chains. This substitution results in the formation of four SiO₃C tetrahedra, as shown. Although not shown here, the structure near the defect is expected to relax in response to this substitution.