Illite layers

Ca-Rb equilibria. The increasing number of high energy sites (table VI) is associated with the formation of collapsed (10 A) illite layers, which were observed (102-104) with increasing number of H.D. cycles. 

Figure 2. Plot of XRD peak positions (CuK radiation ethylene glycol-solvated samples) for Kinney smectite treated with 0.05 N Na + K exchange solutions. Experimental points are labeled with percentages of K in solution. The graph, used to determine percentage illite layers and glycol-spacing for illite/smectites having crystallite thickness of 1-14 layers, is from (42).

Table IV. Percentage of Illite Layers and Interlayer Chemistry of K-Smectites Subjected to Wetting and Drying Cycles in Water at 60°C, and Then Exchanged with 0.1 N SrCl2...

Reference Number Sample Number of WD Cycles Number of Sr Exchanges Meq per 100 g Oxide Illite Layers (Percent)... 

A regular decrease in CEC (meq Sr) occurs as the percentage of fixed K and percentage of illite layers increases. 

Table IV). The quantity of illite layers decreased significantly... 

N NaCl exchanges used to study diagenetic I/S (44). Thus, the illite layers remaining in the WD clays after three Sr-exchanges may be of comparable stability to those formed by burial diagenesis. 

Figure 5. Percentage illite layers versus layer charge for K-smectites subjected to 100 WD cycles in water at 60°C and 1 Sr-exchange. Numbers in parentheses refer to percentage of octahedral charge. Best fit line is for montmorillonites having 69% or more octahedral charge. Data from Tables III and IV.

Figure 6. Stability of illite layers formed by WD mechanism percentage of change in percentage illite layers between 1 and 3 Sr-exchanges is plotted against Of, the angle of tetrahedral rotation. Data from Table V.

Shaking in water prior to each drying cycle speeds reaction. For example, a K-Kinney smectite subjected to 64 WD cycles produced 42% illite layers, with shaking, compared with 30% for K-Kinney subjected to 50 WD cycles, and 32% for K-Kinney subjected to 75 WD cycles, without shaking. 

Figure 7. Kinetics of WD illitization percentage illite layers versus number of WD cycles for Ferruginous and Kinney smectites after 1 Sr-exchange. Data from Table IV.

FIXED INTERLAYER CATIONS PER ILLITE LAYER (C) (EQUIVALENTS PER HALF - UNIT CELL)... 

Figure 8. Percentage illite layers versus equivalents of fixed interlayer cations (Na + K) per illite layer [based on 01o(0H)2]. Solid circles = aluminous smectites with 1 Sr-exchange. Open circles = aluminous smectites with 2 or 3 Sr-exchanges. X = iron-rich smectites with 1 Sr-exchange. Points calculated from data in Tables III, IV, VI, and VIII.

The pattern in Figure 8 is distinct from that for randomly interstratifled I/S produced from bentonite by burial diagenesis (Figure 9). Fixed interlayer cation content for the latter clays is relatively constant at about 0.55 equivalents per illite layer for clays that contain less than 50% illite layers (53). 

Effect of Temperature. Temperature had little effect on the percentage of illite layers formed from K-Kinney, which was subjected to as many as 6 WD cycles at 30°, 60°, and 90°C, and then saturated twice with 0.1 N SrCl2 (Table Vlll). Unfortunately, the experimental products were not X-rayed after a single Sr-saturation therefore, results in Table VIII are not directly comparable to those in Table IV. The erratic data resulting from the 90°C experiments are unexplained. 

Reaction with K-Bearing Minerals. WD experiments with mixtures of Na-Kinney and sparingly soluble K-minerals were undertaken to simulate natural conditions. When K-feldspar was shaken with Na-Kinney at room temperature without WD for as long as 1 year, no illite layers were found in the experimental product. Nor were illite layers formed when muscovite was shaken with Na-Kinney for... 

The experiments also indicate that WD may be an important mechanism for producing I/S at low temperatures in nature by a transformation mechanism (56). The percentage of illite layers formed by this mechanism is proportional to the number of WD cycles, and to the layer charge of the original smectite. Simple K-exchange does not produce stable illite layers in smectite therefore, these layers probably form by WD prior to deposition in subaqueous environments. The exception is found in high pH environments where illite layers may form without WD by chemical reaction, as has been reported previously for alkaline lakes (64, 65). 

Illite layers form relatively quickly by WD (most in less than 20 WD cycles), and the reaction rate is not affected greatly by changes in solution compositions or temperatures that are typical of near-surface environments. Thus, that which has been studied in the laboratory also may occur abundantly in nature. 

Table IX. Percentage of Illite Layers Found in Na-Kinney Subjected...

Frequency plots of the 001/002 ratio of 249 Paleozoic shale (Weaver,1965) and 149 soil illites (White, 1962) indicate that well-ordered 10A 2M illites have a K20 content on the order of 9—10%. Mehra and Jackson (1959) have presented data which suggest that the completely contracted illite layers in illites have 10% K20. It seems likely that well-organized 10A illite layers contain 9—10% K20 and values less than this indicate the presence of non-illite layers or interlayer cations other than potassium. 

Trioctahedral illites have been reported by Walker (1950) and Weiss et al.(1956). Walker s analysis, which he considers only a rough approximation, is given in Table XI. The clay biotite occurs in a Scottish soil and is believed to be authigenic however, it weathers so easily to vermiculite that unweathered material is difficult to find. Due to its instability, it is not likely that much clay-sized biotite exists although trioctahedral biotite-like layers may occur interlayered with dioctahedral illite layers. Such interlayering has been reported by Bassett (1959). 

Random mixed-layer illite-montmorillonite is by far the most abundant of the mixed-layer clays. These two types of layers occur intergrown in all proportions. Tables XLV1 and XLVII contain chemical data for a series of mixed-layer illite-montmorillonites with expanded layers ranging from less than 10% to 60%. When expandable layers comprise more than 60% and less than 10% it is difficult to accurately determine the number of illitic layers. Further, Hower (1967) has shown that many of these clays are partially regularly and partially randomly interstratified. This further complicates interpretation of the X-ray data. 

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