Brucite

Brucite forms through dedolomitization by thermal metamorphism see reaction (2.10)  

It can also form through thermal decomposition of magnesite to form peri-clase (MgO), reaction (2.11), which in both cases is subsequently hydrated to form the hydroxide, reaction (2.12)  

Brucite is a rare mineral, and commercial deposits are only found in China, Russia, and the United States. 

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Fig. 15. A typical powder pattern with three phases Calcite (—), Aragonite and Brucite ( ). The lines below the peaks are the powder lines...

Chry sotile is a hydrated magnesium siHcate and its stoicliiometric chemical composition may be given as AIg2Si20 (0H)4 [12001 -29-5]. However, the geothermal processes wliich ield the chry sotile fiber formations usually involve the co-deposition of v arious other minerals. Tliese mineral contaminants comprise brucite [1317-43-7] (AIg(OH)2), magnetite [1309-38-2] (Fe O, calcite [13397-26-7] (CaCO ), dolomite [16389-88-1] (AIg,CaC02),... 

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... 

Wiih partial replacement of octahedral A1 by Mg and with adsorbed cations. With partial rcplacemcni of octahedral Mg by A1 and with adsorbed cations. That is, regularly alternating talc-like and brucite-like sheets. 

Talk-erde,/. magnesia magnesite, glimmer, m. mica, -hydrat, n. (A/in.) brucite. talkig, a. talcose, talcous. 

Wasser-suppe, /. water gruel. -talk, m. brucite. -teilchen, n. water particle, -tiefe, /. depth of water draft (of ships), -tier, n. water animal, aquatic animal. 

Thus brucite (Mg(OH)2) is also commonly found on surfaces under cathodic protection in seawater. Because more hydroxyl ions (higher pH) are required to cause magnesium hydroxide to precipitate, the magnesium is virtually always found in the calcareous deposits associated with calcium and its presence is an indicator of a high interfacial pH and thus either high cathodic current densities or relatively poor seawater refreshment. 

Figure 2. The brucite structure of Ni(OH), (a) hexagonal brucite layer, in which the small circles are the Ni atoms and the large circles the O atoms and alternate O atoms are below and above the plane of the Ni atoms (b) stacking of the planes showing the orientation of the O-H bonds.

Pandya et al. have used extended X-ray ascription fine structure (EXAFS) to study both cathodically deposited -Ni(OH)2 and chemically prepared / -Ni(OH)2 [44], Measurements were done at both 77 and 297 K. The results for / -Ni(OH)2 are in agreement with the neutron diffraction data [22]. In the case of -Ni(OH)2 they found a contraction in the first Ni-Ni bond distance in the basal plane. The value was 3.13A for / -Ni(OH)2 and 3.08A for a-Ni(OH)2. The fact that a similar significant contraction of 0.05A was seen at both 77 and 297K when using two reference compounds (NiO and / -Ni(OH)2) led them to conclude that the contraction was a real effect and not an artifact due to structural disorder. They speculate that the contraction may be due to hydrogen bonding of OH groups in the brucite planes with intercalated water molecules. These ex-situ results on a - Ni(OH)2 were compared with in-situ results in I mol L"1 KOH. In the ex-situ experiments the a - Ni(OH)2 was prepared electrochemi-cally, washed with water and dried in vac-... 

During the reaction, protons are extracted from the brucite lattice. Infrared spectra [24, 25, 31] show that during charge the sharp hydroxyl band at 3644 cm" disappears. This absorption is replaced by a diffuse band at 3450 cm"1. The spectra indicate a hydrogen-bonded structure for ft-NiOOH with no free hydroxyl groups. ft-NiOOH probably has some adsorbed and absorbed water. However, TGA data... 

Allmann found that when suitable trivalent ions were introduced during the precipitation of the hydroxides of Mg, Zn, Mn, Fe, Co, and Ni, these were incorporated in the lattice and the structure changed from the brucite (Mg(OH)2) to the pyroaurite ([Mg6Fe2(OH)l6] [CO,- 4H20]) type of structure [68], One of the nickel materials he prepared was an Ni/Al hydroxide. Axmann et al. [69-71] have given the nickel compounds the general formula... 

In the pyroaurite structure the brucite layers are cationic. However, on oxidation the resultant brucite layers in y - NiOOH are anionic. To preserve electroneutrality, cations and anions are exchanged in the intercalated layer during the oxidation-reduction process. This is illustrated in Fig. 4. In the case of Mn-substituted materials, some Mn can be reduced to Mn(II). This neutralizes the charge in the brucite layer this part of the structure reverts to the P - Ni(OH)2 structure and the intercalated water and anions are expelled from the lattice. With this there is a concomitant irreversible contraction of the interlayer spacing from 7.80 to 4.65A [72]. 

Under alkaline BW conditions, if magnesium bicarbonate is not removed by softening or other pretreatment processes, it forms brucite, an insoluble flocculant sludge of magnesium hydroxide [Mg(OH)2],... 

Magnesium hydroxide Brucite Mg(OH)4 Common flocculated boiler sludge. 

The BW pH level should be above 10.2 to ensure that hydroxyapatite preferentially precipitates. Also, where magnesium is present, the pH level should be above 10.5 so that magnesium hydroxide [brucite, Mg(OH)2] precipitates, rather than the stickier and more adherent magnesium phosphate [Mg3(P04)2]. In any event, where possible, it is... 

Magnesium typically is present as the phosphate [magnesium hydrox-yphosphate, 3Mg3(P04)2Mg(0H)2] or the hydroxide [brucite, Mg(OH)2] in boiler section soft sludges, and in hard deposits as the silicate (serpentine, 2Mg0-2Si022H20). 

Several other hydroxides of divalent metals crystallize in the same Cdl2 type structure as brucite, notably [610] those of Ca2+,Mn2+, Fe2+, Co2+, Ni2+ and Cd2+. The rates of dehydroxylation of these solids have, how-... 

Fig. 3. A tetrahedral layer in which all the tetrahedra point in the same direction. Fig. 4. A complete layer of octahedra (brucite layer).

Two classes of clays are known [3] (i) cationic clays (or clay minerals) that have negatively charged alumino-silicate layers balanced by small cations in the interlayer space (e.g. K-10 montmorillonite) and (ii) anionic clays which have positively charged brucite-type metal hydroxide layers balanced by anions and water molecules located interstitially (e.g. hydrotalcite, Mg6Al2(0H)igC034H20. 

In the OH series, two phases were detectable by XRD in the dried precipitate. One was a phase with the pyroaurite structure, carbonate having presumably arisen from atmospheric CO2, and the other brucite, Mg(OH)2, 0 which pyroaurite is closely related structurally. For both the CCP and IP series, the only structure identifiable in the dried precipitate was that of magnesium hydroxy carbonate. X-ray analysis of the calcined precursors showed MgO together with y-Fe203 in the case of the OH series and HT, but -Fe203 with the CCP and IP series. MgFe204 spinel was also detectable in some cases. 

Hydrotalcite clays, for example, are built up of positively charged brucite layers, for reviews see Cavani et al. (1991). Upon calcination they become active as solid bases in e.g. aldol and Knoevenagel condensations (see Fig. 2.26) (Fgueras et al., 1998 Corma and Martin-Aranda, 1993 Climent e/a/., 1995). 

Oxides and hydroxides Cuprite, uraninite, baddeleyite, corundum, haematite, rutile, cassiterite, brucite, diaspore, goethite, limonite... 

Among hydroxides such as Mg(OH)2 (brucite) and Ca(OH)2 the packing of the O atoms deviates from an ideal hexagonal closest-packing in that the layers are somewhat flattened the bond angles M-O-M in the layer are larger than the ideal 90° for undistorted octahedra (e.g. 98.5° in Ca(OH)2). 

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