Effects of pressure on order-disorder reactions

Order-disorder phenomena, which play a key role in the energetics, crystal chemistry, and physical properties of numerous solids, may be strongly affected by pressure. The effect of pressure on the state of cation order depends fundamentally on the volume of disordering, ΔV_din = V_disordered - V_ordered. Although ΔV_dis may be positive or negative, it is usually positive for cation ordering in rock-forming minerals: pressure, therefore, tends to increase order in geological systems. Three structural mechanisms that contribute to ΔV_dis are (1) positive ΔV_dis (typically ≤ 1%) associated with ordering of similar cations over two or more similar sites, (2) variable ΔV_dis (-1% ≤ ΔV_dis ≤ 1%) associated with Al-Si tetrabedral ordering in framework minerals, and (3) large ΔV_dis (up to 5%) associated with cation ordering involving mixed valence, mixed coordination, or both. Pressure alters order-disorder reaction rates by restricting cation motions or changing cation diffusion mechanisms. Influences of pressure on ordering in olivines, orthopyroxenes, high-pressure ferromagnesian silicates, spinels, garnets, perovskites, and rhombohedral carbonates are discussed. Several experiments are proposed to elucidate the phenomenon of high-pressure ordering in systems of mineralogical interest.