Characterization of neoformed illite from hydrothermal experiments at 250 C and P{sub v,soln}: An HRTEM/ATEM study
- Washington State Univ., Pullman, WA (United States). Dept. of Geology
Solid products from hydrothermal experiments conducted at 250 C and P{sub v,soln} were characterized by powder X-ray diffraction (XRD) and ATEM/HRTEM. Experiments were conducted with muscovite, kaolinite, and quartz or amorphous silica in 2M KCl solutions for 43 to 176 d. Post-experiment solution compositions lie either within the illite(0.88 K) stability field or on the illite(0.88 K)-kaolinite or illite(0.88 K)-diaspore univariant boundaries in log (a{sub K}{sup +}/a{sub H}{sup +}) vs. log a{sub H{sub 4}SiO{sub 4}} activity space. Transmission electron microscopy (TEM) observations of muscovite grain edges reveal the neoformation of illite crystals with a range of compositions (ATEM) from 0.31 to 0.89 K/O{sub 10}(OH){sub 2}. The range of K-contents appears to narrow toward 0.88 K/O{sub 10}(OH){sub 2} with increased experiment duration. HRTEM suggests the presence of 2 to 11 layer fundamental particles composed of illitic layers with 10 A periodicity. Fundamental particle thicknesses increase toward an average of 8 layers/particle with increased experiment duration. In the longer duration experiments, fundamental particle thicknesses were normally distributed about thicknesses of 4 and 8 layers, whereas fundamental particles with thicknesses <4 layers were common in a shorter duration experiment. The compositions and structure of the illites are consistent with the multiphase model, which states that the smectite-to-illite transition occurs through the step-wise formation of solubility-controlling phases consisting of fundamental particles with thicknesses of 1, 2, 4, and {le}8 layers. The increase in K-content and fundamental particle thickness with the extent of reaction suggests that the illite crystals underwent a prograde reaction culminating in the formation of end-member illite [0.88 K/O{sub 10}(OH){sub 2}]. This reaction, in conjunction with the previously observed, retrograde reaction from muscovite to end-member illite, demonstrates the stability of end-member illite in the system K{sub 2}O-Al{sub 2}O{sub 3}-SiO{sub 2}H{sub 2}O at 250 C.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- OSTI ID:
- 343756
- Journal Information:
- American Mineralogist, Vol. 83, Issue 11-12Pt1; Other Information: PBD: Nov-Dec 1998
- Country of Publication:
- United States
- Language:
- English
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