A compound containing hydroxide anions in association with two or more metal cations. MMH particles are extremely small and carry multiple positive charges. They can associate with bentonite to form a strong complex that exhibits highly shear-thinning properties, with high and fragile gel strengths, high yield point (YP), and low plastic viscosity (PV). MMH is described as a mixed-metal layered hydroxide (MMLH). In the crystal layers, Al⁺³ , Mg⁺² and OH^- ions reside, but due to symmetry considerations, there is not enough room for sufficient OH^- ions to electrically offset the charges of the two cations. Therefore, a net positive charge exists on the crystal surfaces. Exchangeable anions sit on the positive surface (much the same as cations sit on negative clay surfaces). MMH muds are used as nondamaging drilling fluids, metal-reaming fluids (to carry out metal cuttings) and for wellbore shale control. Being cationic, MMH mud is sensitive to anionic deflocculants and small anionic polymers, such as polyphosphates, lignosulfonate or lignite.

Reference:
Burba JL III and Crabb CR: "Laboratory and Field Evaluation of Novel Inorganic Drilling Fluid Additive," paper IADC/SPE 17198, presented at the IADC/SPE Drilling Conference, Dallas, Texas, USA, February 28-March 2, 1988.

Fraser L and Enriquez F: "Mixed-Metal Hydroxides Fluid Research Widens Applications," Petroleum Engineer International 64, no. 6 (June 1992): 43-46.