Magnesium hydroxychloride is the traditional term for several chemical compounds of magnesium, chlorine, oxygen, and hydrogen whose general formula xMgO·yMgCl2·zH2O, for various values of x, y, and z; or, equivalently, Mgx+y(OH)2xCl2y(H2O)z−x. The simple chemical formula that is often used is Mg(OH)Cl, which appears in high school subject, for example.Other names for this class are magnesium chloride hydroxide, magnesium oxychloride, and basic magnesium chloride. Some of these compounds are major components of Sorel cement.
Compounds
The ternary diagram of the system MgO – MgCl2 – H2O has the following well-defined and stable phases:
Mg(OH)2 (magnesium hydroxide, the mineral brucite) 2Mg(OH)2·MgCl2·4H2O = Mg3(OH)4Cl2·4H2O ("phase 2", "2:1:4") 3Mg(OH)2·MgCl2·8H2O = 2Mg2(OH)3Cl·4H2O ("phase 3", "3:1:8") 5Mg(OH)2·MgCl2·8H2O = 2Mg3(OH)5Cl·4H2O ("Phase 5", "5:1:8") 9Mg(OH)2·MgCl2·5H2O = Mg10(OH)18Cl2·5H2O ("Phase 9", "9:1:5") MgCl2·6H2O (magnesium chloride hexahydrate) Phase 3 and phase 5 may exist at ambient temperature, whereas the phase 2 and phase 9 are stable only at temperatures above 100 °C. All these compounds are colorless crystalline solids. At ambient temperature, there are also gel-like homogeneous phases that form initially when the reagents are mixed, and eventually crystallize as phase 5, phase 3, or mixtures with Mg(OH)2 or MgCl2·6H2O. There are also other lower hydrates that can be obtained by heating the "natural" phases:
2Mg(OH)2·MgCl2·2H2O (phase 2 dihydrate; ~230 °C) 3Mg(OH)2·MgCl2·5H2O (phase 3 pentahydrate; ~110 °C) 3Mg(OH)2·MgCl2·4H2O (phase 3 tetrahydrate; ~140 °C) 5Mg(OH)2·MgCl2·4H2O (phase 5 tetrahydrate; ~120 °C) 5Mg(OH)2·MgCl2·3H2O (phase 5 trihydrate; ~150 °C) 9Mg(OH)2·MgCl2·2H2O (phase 9 dihydrate; ~190 °C) In addition, a heptahydrate of phase 5, 5Mg(OH)2·MgCl2·7H2O, can be obtained by washing the natural octahydrate with ethanol. All four stable phases have anhydrous versions, such as 3Mg(OH)2·MgCl2 (anhydrous phase 3) and 5Mg(OH)2·MgCl2 (anhydrous phase 5), with the crystal structure of Mg(OH)2. They can be obtained by heating them to about 230 °C (phases 3 and 5) about 320 °C (phase 2), and about 260 °C (phase 9).
History These compounds are the primary components of matured magnesia cement, invented in 1867 by the French chemist Stanislas Sorel. In the late 19th century, several attempts were made to determine the composition of set Sorel's cement, but the results were not conclusive. Phase 3 was properly isolated and described by Robinson and Waggaman in 1909, and phase 5 was identified by Lukens in 1932.
Properties
Solubility The oxychlorides are only very slightly soluble in water. In the system MgO – MgCl2 – H2O at about 23 °C, the completely liquid region has vertices at the following triple equilibrium points (as mass fractions, not molar fractions):
S1 = 0.008 MgO + 0.170 MgCl2 + 0.822 H2O (Sol:Mg(OH)2:P5) S2 = 0.010 MgO + 0.222 MgCl2 + 0.768 H2O (Sol:P5:P3) S3 = 0.012 MgO + 0.345 MgCl2 + 0.643 H2O (Sol:P3:MgCl2·6H2O) The other vertices are pure water, magnesium chloride hexahydrate, and the saturated Mg(OH)2 solution (0.0044 MgO + 0.9956 H2O by mass).
Decomposition and degradation The anhydrous forms decompose when heated above 450-500 °C by decomposition of the hydroxide and chloride anions, releasing water and hydrogen chloride and leaving a magnesium oxide residue, by the reactions:
2 OH− → O2− + H2O H2O + 2 Cl− → O2− + 2 HCl Extended exposure of magnesium oxychlorides to water leaches out the soluble MgCl2, leaving hydrated brucite Mg(OH)2. On exposure to the atmosphere, the oxychlorides will slowly react with carbon dioxide CO2 from the air to form magnesium chlorocarbonates. Anhydrous and partially hydrated forms also absorb water, turning into phase 5 and then phase 3 on the way to the chlorocarbonate. The exceptions are the dihydrate and hexahydrate of phase 9, that remain unchanged for many months.
Structure The crystal structure of phase 3 is triclinic with space group P 1 ¯ {\displaystyle P{\bar {1}}} and z = 2. The solid consists polymeric aquohydroxo cations, in the form of double chains of magnesium atoms surrounded and bridged by the oxygen atoms in hydroxy groups and complexed water molecules. These linear cations are interleaved and neutralized by chloride anions and some unbound water molecules, yielding the general formula [(Mg2(OH)3(H2O)3)n]n+(Cl−)n(H2O)n. The structure of phase 5 is believed to be similar, with generic formula [(Mg3(OH)5(H2O)x)n]n+(Cl−)n(H2O)n(4−x). The anhydrous forms of phase 3 and phase 5 have the same structure as Mg(OH)2: namely, layers of magnesium cations, each sandwiched between two layers of hydroxy or chloride anions. Phase 5 crystals form as long needles consisting of rolled-up sheets. The Raman spectrum of phase 3 has peaks at 3639 and 3657 cm−1, whereas phase 5 has peaks at 3608 and 3691 cm−1, and brucite has a peak at 3650 cm−1. These peaks are attributed to stretching vibrations of the OH groups. Phase 3 has also a peak at 451 cm−1, attributed to the stretching of Mg–O bonds.
Preparation
From MgO or Mg(OH)2 and MgCl2 Phases 3 and 5 can be prepared by mixing powdered magnesium oxide MgO with a solution of magnesium chloride MgCl2 in water H2O, in molar ratios 3:1:11 and 5:1:13, respectively, at room temperature. This is the common method of preparing Sorel magnesia cement. Magnesium hydroxide Mg(OH)2 can also be used instead of the oxide, with adjusted amount of water. For best results, the magnesium oxide should have small particle size and large surface area. It can be prepared by calcination of magnesium hydroxycarbonate Mg5(OH)2(CO3)4·4H2O at about 600 °C. Higher temperatures increase particle size leading to slower reaction rate. It is believed that, during the reaction, the magnesium oxide is continuously hydrated and dissolved, helped by the slightly acidic character of the magnesium chloride solution. The acidity is attributed to hydrolysis of the magnesium hexahydrate cations:
[Mg(H2O)6]2+ ↔ [Mg(OH)(H2O)5]+ + H+ The protons (which are actually hydrated, e.g. as [H3O]+) make the solution acidic; the pH varies from 6.5 to 4.7 as the concentration of MgCl2 increases from 30% to 70% (weight basis). The protons then react with and dissolve the nearly insoluble oxide or hydroxide, by such reactions as
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