Fig. 4: Schematic illustrations of shearing mechanism of Mg₂SiO₄.

a Perspective views of oxygen anion layers, showing how the original α structure (1 atm) was compressed along its a-axis (P), and then recrystallised into the dense structure (D) induced by fast shearing (slipping) motions of these layers at the slip planes shown with brown colour. The original α structure consists of alternating A and B layers of oxygen forming a hexagonal close packing (hcp) arrangement. The dense polymorph structure (β, γ, or ε) all consists of alternating A, B, and C layers of oxygen forming a cubic close packing (ccp) arrangement. b The shearing model toward the γ structure, where cooperative slip motions of the oxygen layers occur along the c axis of the α structure, [001]_α. Only the oxygen anions are shown for clarity. See Supplementary Fig. 4 for the accompanying cooperative motions of magnesium and silicon cations. The unit cell of α structure is indicated by the rectangle with dotted lines. The upper layer B slips with respect to the lower layer A by partial dislocation with Burgers vector b_p = 1/12[013]_α, as indicated by the thick solid pink arrow. The other partial dislocations were indicated by the thin solid pink arrow, where all these partial dislocations in total constitute the perfect dislocation with the Burgers vector b_t = [001]_α that defines the direction of macroscopic shearing along the c axis^17,26. c The shearing model toward the ε structure, where cooperative slip motions of the oxygen layers occur along the b axis of the α structure, [010]_α. The unit cell of α structure is shown by the rectangle with dotted lines. The layer B slips with respect to the layer A by partial dislocation with b_p = 1/3[010]_α, as indicated by the thick solid pink arrow. The other partial dislocations were indicated by the thin solid pink arrow, where all these partial dislocations in total constitute the perfect dislocation with b_t = [010]_α that defines the direction of macroscopic shearing along the b axis¹⁷.