Tachyhydrite is an unstable mineral, a hydrous chloride of calcium and magnesium with formula: CaMg2Cl6·12H2O.

Upon exposure to moist air it rapidly deliquesces and dissolves.

It forms a colorless to yellow trigonal crystal with a vitreous luster.

It has good cleavage in three directions and typically occurs in crystalline masses.

[4] According to a patent filed years ago by a Halliburton researcher, high strength hydrochloric acid treatment of magnesium containing carbonates creates tachyhydrite, which will seal the rock pores and inhibit oil flow unless washed with a light acid or water before and after the treatment.