GPa after which it transforms to phase of triclinic symmetry. Undecahydrate The undecahydrate , meridianiite, is stable at atmospheric pressure only below 2 °C. Above that temperature, it liquefies into a mix of solid heptahydrate and a saturated solution. It has a eutectic point with water at −3.9 °C and 17.3% (mass) of . Large crystals can be obtained from solutions of the proper concentration kept at 0 °C for a few days. At pressures of about 0.9 GPa and at 240 K, meridianiite decomposes into a mixture of ice VI and the enneahydrate . Enneahydrate The enneahydrate was identified and characterized only recently, even though it seems easy to produce (by cooling a solution of and sodium sulfate in suitable proportions). The structure is monoclinic, with unit-cell parameters at 250 K: a = 0.675 nm, b = 1.195 nm, c = 1.465 nm, β = 95.1°, V = 1.177 nm3 with Z = 4. The most probable space group is P21/c. Magnesium selenate also forms an enneahydrate , but with a different crystal structure. Natural occurrence As and ions are respectively the second cation and the second anion present in seawater after and , magnesium sulfates are common minerals in geological environments. Their occurrence is mostly connected with supergene processes. Some of them are also important constituents of evaporitic potassium-magnesium (K-Mg) salts deposits. Bright spots observed by the Dawn Spacecraft in Occator Crater on the dwarf planet Ceres are most consistent with reflected light from magnesium sulfate hexahydrate. Almost all known mineralogical forms of are hydrates. Epsomite is the natural analogue of "Epsom salt". Meridianiite, , has been observed on the surface of frozen lakes and is thought to also occur on Mars. Hexahydrite is the next lower hydrate. Three next lower hydrates – pentahydrite, starkeyite, and especially sanderite – are rare. Kieserite is a monohydrate and is common among evaporitic deposits. Anhydrous magnesium sulfate was reported from some burning coal dumps. Preparation Magnesium sulfate is usually obtained directly from dry lake beds and other natural sources. It can also be prepared by reacting magnesite (magnesium carbonate, ) or magnesia (oxide, MgO) with sulfuric acid (). Another possible method is to treat seawater or magnesium-containing industrial wastes so as to precipitate magnesium hydroxide and react the precipitate with sulfuric acid. Also, magnesium sulfate heptahydrate (epsomite, ) is manufactured by dissolution of magnesium sulfate monohydrate (kieserite, ) in water and