Minerals brucite, Mg

The structure of the mineral brucite Mg(OH)2 (space group P2>m ) is formed by Mg(OH)g octahedra, which are connected by edges, forming layers parallel to the (001) plane (see Fig. 7). Hydroxyl groups OH lie on the 3-fold axes and each H-atom is equidistant from three oxygen atoms of the next layer, which is not a perfect position for forming hydrogen bonds between layers. 

A formal description of a mineral presents all the physical and chemical properties of the species. In particular, distinctive attributes that might facilitate identification are noted, and usually a chemical analysis of the first or type specimen on which the name was originally bestowed is included. As an example, the complete description of the mineral brucite (Mg(OH)2), as it appears in Dana s System of Mineralogy, is presented as Appendix 3. Note the complexity of this chemically simple species and the range of information available. In the section on Habit (meaning shape or morphology) both acicular and fibrous forms are noted. The fibrous variety, which has the same composition as brucite, is commonly encountered (see Fig. I.ID) and is known by a separate name, nemalite. ... 

If the octahedral layers are made up of three Mg + coordinated with six or OH, then all the cation sites are occupied by Mg. The layers are then termed trioctahedral, or brucite layers, because they are compositionally equivalent to the mineral brucite, Mg(OH)2. 

Chlorites [(Mg,Fe)3(Si,Al)Pjg(OH)2 (Mg,Fe)3(OH)J have a 2 1 sandwich structure. They consist of Mg (or) Al - octahedra sandwiched between 2 silica (or) alumina tetrahedra. The interlayer space between chlorite layer is composed of (Mg, Fe )(OH)g, which has a closer resemblance to the mineral brucite (Mg(OH)2). 

The mineral brucite, Mg(OH)2, adopts a Cdl2-type structure, and substitution of for results... 

Magnesium oxide. The natural minerals, i.e., magnesite (MgCO ), brucite [Mg(OH)9], etc., after being crushed to predetermined size, are calcined at temperatures varying from 1055 to 2000 K, depending upon whether a caustic or a dead-burned produc t (periclase) is being... 

The fluid contains arbitrarily small amounts of Ca++, Mg++, and HCO3, as is necessary in order for the program to be able to recognize dolomite. The initial magnesium content is set small to assure that the hydroxide mineral brucite is not supersaturated in the alkaline fluid. 

This compound is isostructural with brucite (Mg(OH)2) and Cdl2. The unit cell is hexagonal with a = 0.3258 nm and c = 0.4605 nm. The structure consists of sheets of corner-sharing, trigonally distorted Fe(OH)6 octahedra stacked along the [001] direction. The Fe" ions occupy only half the available octahedral interstices and this results in a structure in which each filled layer of sites alternates with an empty layer of sites. The OH radical behaves as a single entity. Amakinite is a rare mineral of the composition (Fe,Mg,Mn)(OH)2, also with brucite structure. Fe(OH)2 is readily oxidized by air and even by water, upon which the colour changes from white to brownish. The structure can be maintained up to a replacement of one tenth Fe" by Fe " (Bernal et al., 1959). 

Hydrotalcite is a natural mineral of ideal formula Mg6Ab.(()H)1, iCO . 4H20, having a structure similar to brucite, Mg(OH)2. In hydrotalcite the Mg cations are partially replaced with Al3+ and the resulting positive charge is compensated by anions, typically carbonate, in the interlamellar space between the brucite-like sheets. When hydrotalcite is calcined at ca. 500 °C it is decarbonated and dehydrated to afford a strongly basic mixed Mg/Al oxide. Rehydration restores the original hydrotalcite structure and creates Bronsted base sites (OH ) in the interlamellar space. 

Hydrotalcites, or layered double hydroxides (LDH), are a class of material related to the mineral hydrotalcite which has the formula [Mg6Al2(0H)i6]C03-4H20. They are layered double hydroxides with the general formula M M]l ,(0H)2,A zH20 which have the trivalent and divalent cations contained within the octahedral sites of posihvely charged hydroxide sheets, in a stmcture akin to that of brucite, Mg(OH)2. The anions, A , and water are contained between the layers of sheets and their intercalation and removal can be monitored by the position of the (003) refiection in their powder diffraction patterns [56]. A schematic of the structure is shown in Figure 21.15 [56]. 

In the case of MgO, various types of OH groups can form, depending on the sites involved in the chemisorption process. In a periodic ab initio molecular dynamics study [113], it has been shown that the faces which are energetically unfavorable in the clean MgO surface may become favored in the presence of water. The calculations indicated that there is a net stabilization upon going from the MgO(lOO) surface covered with molecularly adsorbed water to the MgO(lll) surface covered with dissociated water. This is not too surprising if we consider that brucite, Mg(OH)2, is a mineral with hexagonal structure which exhibits the (0001) face as the most stable one and that MgO is often produced by thermal decomposition of brucite. 

At low temperature the ferrous-iron-rich brucite (Mg,Fe)(OH)2 can subsequently disproportionate into a magnesium end-member brucite and magnetite, which becomes the stable iron oxide in contact with water at low temperature. The formation of magnetite under certain conditions can result in the release of hydrogen gas. At elevated temperatures in hydrothermal and meta-morphic reactions (Equations (2) and (3)), magnesium or potassium can be removed from solution forming the minerals chlorite or sericite ... 

The primary mineral, chlorite, which occurs in rocks as large crystals, possesses an interlayer sheet composed largely of Mg(OH)2. Since the mineral brucite is composed of magnesia sheets with the same basic structure, the single interlayer sheet in chlorite is termed the brucite layer. Isomorphous substitution of part of the Mg by produces a positively chained hydroxide sheet (see formula above) that props the 2 1 layers apart at a c-spacing of 14 A. This rigid interlayer contrasts with the hydrated interlayer of vermiculite, and even though chlorite and vermiculite have similar c-... 

The partial pressure of water vapour in solar composition material depends on several factors. Formation of anhydrous minerals consumes about 23% of total oxygen. The formation of hydrous minerals, such as glaucophane, tremolite, serpentine, and talc consumes more water vapour. For example, serpentine and brucite Mg(OH)2 form by the hydration of forsterite Mg2SiC>4 grains by water vapour... 

Magnesium is mined in significant quantities from the minerals dolomite (CaMg(C03)2) and magnesite (MgC03) and in smaller quantities from brucite (Mg(OH)2) and carnallite (KMgCl3 x 6(H20)). 

Other compounds crystalhzing with a Cdl2 lattice include MgBr2, Mgl2, Cal2, iodides of many J-block metals, and many metal hydroxides including Mg(OH)2 (the mineral brucite) in which the [OH] ions are treated as spheres for the purposes of structural description. 

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