Tachyhydrite is an unstable mineral, a hydrous chloride of calcium Mg. It is a rare component of marine evaporite salt deposits. Upon exposure to moist air it rapidly deliquesces and dissolves.
It forms a colorless to yellow trigonal crystal with a vitreous luster. It is soft with a Mohs hardness of 2 and has a low specific gravity of 1.66. It has good cleavage in three directions and typically occurs in crystalline masses.
It was first described in 1856 for an occurrence in Stassfurt , Saxony. Its name is from the Greek for quick water, in reference to its ready deliquescence.
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.