Chlorite layer charge

Fig. 2.13 The 1 1 and 2 1 layer arrangements in the sheet structure minerals and the (010) view of the structures of the serpentine, clay, talc, pyrophyllite, mica, and chlorite minerals. X = layer charge per formula unit. [From Bailey (1980), Fig 1.1, p. 3 Fig 1.2, p. 6.1...

The negative relation between K2 O and Fe2 03 also occurs in the illites and has been discussed. Fe203 is positively correlated with H20+ and presumably negatively with layer charge. H20+ (greater than 110°C) and H20 (less than 110°C) are inversely related. These correlations suggest some of the H20+ may be trapped interlayer water not easily released (at 110°C) from the clays with a high proportion of contracted layers or that chloritic layers increase as the proportion of expandable layers decrease. 

When all the K is assigned to the contracted 10 A layers and a plot made of the amount of K per 10 A layer versus percent 10 A layers (Fig.18), a distinct positive relation is apparent. The data indicate that when there are no expanded layers the clay contains 0.8 K per O10(OH)2 the amount of K systematically decreases and for 40% contracted layers the concentration is 0.55 K per Oi 0(OH)2. If some K is present in the expanded layers the amount of K in the contracted layers would be even lower. These data may indicate the calculated proportion of contracted layers is high or, on the basis of Hower s (1967) reasoning, the amount of ordered interlayering increases with a decrease in the proportion of contracted 10 A layers (less average charge per layer is required to contract ordered interlayers than random interlayers). Another possibility is that approximately 20% of the contracted layers are chlorite. It is difficult to detect less than 40% illite layers interlayered with montmorillonite and it is probably equally difficult to detect 20% or so chloritic layers (Weaver and Beck, 1971a). 

K is obtained from associated K-feldspars and micas. The layer charge is increased by the reduction of iron in the octahedral sheet and incorporation of Al, entering through the ditrigonal holes in the basal oxygen plane, into the tetrahedral sheets (Weaver and Beck, 1971a Pollard, 1971). Weaver and Beck have presented evidence that indicates mixed-layer clays formed in this manner contain 20—30% chloritic layers and are actually mixed-layer illite-chlorite-montmorillonite clays. 

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. 

Foster found two series of ionic replacements in the chlorites, replacement of Mg by Fe " " and replacement of tetrahedral and octahedral Al by Si and Mg. Replacement of Mg by Fe in the octahedral sheets is ion for ion and does not cause any change in the layer charges. 

Vermicuhte is an expandable 2 1 mineral like smectite, but vermiculite has a negative charge imbalance of 0.6—0.9 per 02q(0H)2 compared to smectite which has ca 0.3—0.6 per 02q(0H)2. The charge imbalance of vermiculite is satisfied by incorporating cations in two water layers as part of its crystal stmcture (144). Vermiculite, which can be either trioctahedral or dioctahedral, often forms from alteration of mica and can be viewed as an intermediate between UHte and smectite. Also, vermiculite is an end member in a compositional sequence involving chlorite (37). Vermiculite may be viewed as a mica that has lost part of its K+, or a chlorite that has lost its interlayer, and must balance its charge with hydrated cations. 

Fig. 13-20.—The sequence of layers normal to the cleavage plane of the chlorite minerals, showing the alternation of mica layers such as [Mg3AlSis-Oio(OH)2] with charged brucitelike layers [MgaAl-(OH),]+.

Brown and Bailey (1962) examined 300 chlorites from different localities and found that approximately 80% had the lib structure but found examples of the orthorhombic lb, monoclinic lb and la structural type. The relative abundance of the polytypes was related to structural stability. Composition influences to some extent the stability of the chlorites through its effect on the cation charge and amount of distortion of the hexagonal network caused by size adjustments. Increasing tetrahedral A1 substitution is accompanied by an increase in octahedral Fe to maintain a reasonable degree of fit between the two types of layers. 

The last group, (d), of structures in which positively and negatively charged layers alternate, is also a small one and includes the chlorite minerals (p. 824) and some hydroxyhalides such as [Na4Mg2Cli2] [Mg7Al4(OH)2 2] (p- 212). 

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