Brucite sheet

Chlorite contains triple layers alternating with brucite sheets [Mg3(OH)(J in the unit area considered Mg occupies all octahedral sites whereas A1 occupies only two-thirds of them]. An ideal formula for a chlorite without A1 would be Mg3(OH)6Mg3Si4Oio(OH)10. In natural chlorites some Mg and some Si are replaced by Al. 

Chlorite can occur as a clay-sized mineral. Most consist of a 2 1 talc layer plus a brucite sheet. This forms a unit 14 A thick. Most chlorites are trioctahedral although a few dioctahedral chlorites have been found. Some chlorites have both dioctahedral and trioctahedral sheets. Because substitution can occur both in the 2 1 layers and in the brucite sheet, the chlorites have a wide range of compositions. The coarser grained chlorites have been analyzed and classified (Hey, 1954) but relatively little is known of the composition of sedimentary chlorites. 

Some of the high Fe values may be real. During weathering under neutral to acid conditions, the Mg-rich brucite sheet tends to be stripped out and removed from the immediate environment. If new material is precipitated between the talc layers, it is more apt to be Fe and Al than Mg. As chlorites go through the sedimentary cycle, perhaps several times, their average Fe and Al content will tend to increase. 

Mg-rich chlorites do not seem to form readily in low-temperature marine environments. Mixed-layer chlorite-vermiculites form fairly easily in magnesium-rich environments but complete development of the brucite sheets must be considerably more difficult. Mixed-layer chlorite-vermiculite is the predominant clay in the Lower Ordovidian limestones and dolomites of southern United States, usually over a thousand feet thick and extending over 500,000 square miles, yet very little chlorite is present (Weaver, 1961a). These mixed-layer clays and others like them appear to have formed on extensive tidal flats where the clays were exposed to alternating evaporitic... 

Vermiculite and vermiculite layers interstratified with mica and chlorite layers are quite common in soils where weathering is not overly aggressive. (A few references are Walker, 1949 Brown, 1953 Van der Marel, 1954 Hathaway, 1955 Droste, 1956 Rich, 1958 Weaver, 1958 Gjems, 1963 Millot and Camez, 1963 Barshad and Kishk, 1969.) Most of these clays are formed by the removal of K from the biotite, muscovite and illite and the brucite sheet from chlorite. This is accompanied by the oxidation of much of the iron in the 2 1 layer. Walker (1949) has described a trioctahedral soil vermiculite from Scotland formed from biotite however, most of the described samples are dioctahedral. Biotite and chlorite with a relatively high iron content weather more easily than the related iron-poor dioctahedral 2 1 clays and under similar weathering conditions are more apt to alter to a 1 1 clay or possibly assume a dioctahedral structure. 

Additional brucite sheets would tend to develop between unaffected sheets and avoid interlayer spaces adjacent to where a brucite sheet had already been organized. Initially, these brucite sheets would be distributed randomly but as they approached the point where they filled approximately half the interlayer spaces, the charge distribution and minimum free-energy requirements would dictate that the final brucite... 

If the silica sheets in a 2 1 Fe-rich clay had enough A1 substitution to adjust to the size of the octahedral sheet, this would most likely provide sufficient layer charge to cause contraction and the formation of a biotite mica, or the bonding of a positively charged brucite sheet and the formation of a chlorite. Low-temperature micas of this composition may well exist, but have not yet been recognized. It is not known if low-temperature chlorites of this composition can form. 

Figure 2. Structure of a typical chlorite-like hydroxy-interlayered clay in which the galleries of a 2 1 smectite structure are filled, or nearly so, with brucite-like sheets of mainly edge-shared Mg(OH) octahedra. Aluminum occasionally substitutes for magnesium in the brucite sheet to provide the charge balance necessary for electrical neutrality.

Chlorites have been studied spectroscopically mainly on account of Fe2+- Fe3+ IVCT bands near 14,300 cm-1 that contribute to their optical spectra (e.g., White and Keester, 1966 Faye, 1968b Smith and Strens, 1976 Smith, 1977). Two other bands centred near 11,500 cm-1 and 9,500 cm-1 provide estimates for the crystal field parameters of Fe2+ ions in chlorite of A0 = 11,200 cm-1 and CFSE = 4,300 cm-1. Crystal spectra of Cr3+-bearing chlorite, kammererite, yield absorption bands at 18,450 cm-1 and 25,000 cm-1, giving A0 = 18,450 cm-1 and a CFSE of 22,140 cm-1 for octahedrally coordinated Cr3+ ions surrounded by OH- ions in the brucite sheets. The spectra of other Cr3+-bearing clay silicates have been described (Calas et al., 1984), including clinochlore and stichtite. 

Figure 3A. Schematic of a brucite sheet showing the relative locations of all Mg2+ by removing the top layer of oxygen. Minerals may contain any combination of Al3+ and Mg2+, while Fe3+ or Fe2+ may also substitute isomorphously (from Taylor and Ashcroft, 1972, with...

Figure 6 Structures of common clay minerals I I Silica tetrahedral sheet I I alumina octahedral sheet I I brucite sheet.

Figure 12.18 Variation in wave-number of the OH elongation vibration of the brucite sheet of a hydrotalcitc as a function of its composition.

Strongly Alkaline, Confined Environments, Because Si is most soluble in the form of silicate anions, it is not surprising that smectites form most rapidly in dilute alkaline solutions of sodium silicate and magnesium chloride. The layer silicate structure evolves by the precipitation of a planar Mg(OH)2 (brucite) sheet, on which monomeric silicate ions condense. Silica polymers are unable to reorganize into layered structures. It seems that the metal hydroxide sheet must be layered to begin with, a fact that necessitates 6-coordination of the metal ion that is to form the octahedral sheet of the 2 1 layer. Smectites can be formed from hydroxide sheets of the following cations (listed in order of increasing radius) ... 

Fig. 7-3. Representative hydrotalcite structure. The charge imparted by Al3+ incorporation into the edgesharing octahedra of the brucite sheets is compensated by charge balancing anions in the interlayer.

Chlorites occur extensively in soils and are 2 1 1 layer silicates (Fig. 5.6). The positively charged and substituted brucite sheet between the negatively charged mica-like sheets restricts swelling, decreases the effective surface area, and reduces the effec-... 

During the reactions leading to chlorite or an authigenic aluminosilicate, protons in particular will be present in excess because abundant hydroxyl groups form part of the brucite sheets of the chlorite. Numerous successions from the Saharan basins offer clear evidence of a decrease in the chlorite content of the reservoir rocks at the expense of the kaolinite. From this we may suspect that a reaction splitting off the acid may have taken place under the effect of a mineral transformation (Boles and Frank 1979). It should be noted that contrary to commonly held notions the leaching to which the shales are subjected represents rather a trap than a source for these cations. 

Gibbsite sheets (Fig. 7.1b) contain octahedral [Al(OH)6] units, where each hydroxide ion is co-ordinated to two Al " cations. Thus, only two-thirds of the hydroxide sites are occupied in bridging two A1 ions. Analogously, brucite sheets, typical of some natural cationic and anionic clays, contain Mg(OH)2 units where each hydroxide ion is bonded to three cations. 

The luminescence spectra of Cr " centers in two chlorite crystals (Fig. 4.176) have been presented (Czaja et al. 2014). Chromium ions occupy the strong crystal-field site M4 in the brucite sheet and the intermediate crystal-field site in the inner... 

Trioctahedral brucite sheet, dioctahedral mica sheet. 

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