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Disordered silica with tridymite-like structure in the Twiggs clay

Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.

View larger version (8K): [in this window] [in a new window]   FIGURE 1. XRD patterns of silica polymorphs: (a) opal-a, (b) opal-ct, (c) opal-c, (d) cristobalite, and (e) tridymite. Q indicates quartz (101) peak superimposed on opal-a pattern. (Modified from Elzea et al. 1994.)

View larger version (18K): [in this window] [in a new window]   FIGURE 2. Location of study area in the Coastal Plain of Georgia. Hachured pattern depicts the outcrop belt of the Twiggs clay and associated units. 1 = type locality. 2 = alternative reference section. (Base modified from Pickering 1976.)

View larger version (61K): [in this window] [in a new window]   FIGURE 3. Centric and colonial diatom forms preserved in silica lepispheres in the Twiggs clay. S = sponge spicule.

View larger version (11K): [in this window] [in a new window]   FIGURE 4. XRD pattern of whole rock sample P10, a representative disordered-silica rich specimen.

View larger version (18K): [in this window] [in a new window]   FIGURE 5. Whole rock diffractogram of sample B20 illustrating a typical 5-peak profile fit of the 19 to 24°2 band. Vertical lines with d and hkl values represent peak positions from IUDD reference files for 18–1170 tridymite (T), 33–1161 quartz (Q), and 13–0135 montmorillonite (S). Numbers 1, 2, 3, 4, and 5 represent individual peaks determined by fitting routine. Smooth, solid black line represents the composite profile formed by peaks 1, 2, 3, 4, and 5. Light gray line near the top of the figure illustrates the residuum, the discrepancy between the modeled curve and the observed curve. "R" is the residuum expressed as a percent.

View larger version (10K): [in this window] [in a new window]   FIGURE 6. Whole rock diffractogram of sample B15 illustrating profile fitof the 19 to 24 °2 band with five peaks (a) and six peaks (b). In b, note the offset of peaks from the locations of tridymite (404) and cristobalite (101) maxima. Also note increased asymmetry of peaks in b relative to a. See Figure 5 for explanation of symbols.

View larger version (11K): [in this window] [in a new window]   FIGURE 7. Whole rock diffractogram of sample B34 illustrating profile fit of the 19 to 24 °2 band with five peaks (a) and six peaks (b). In b, note increased breadth and asymmetry of quartz peak 1. See Figure 5 for explanation of symbols.

View larger version (9K): [in this window] [in a new window]   FIGURE 8. Whole rock diffractogram of sample B13 illustrating profile fit of the 19 to 24 °2 band with five peaks (a) and six peaks (b). In both models, diffraction maxima are close to ideal peak positions; however, tridymite (404) and cristobalite (101) peaks display increased asymmetry in b. See Figure 5 for explanation of symbols.