METHODS/MATERIALS

The mineral standards used for the new bulk powder mixtures are similar to those used during ODP Leg 190: quartz (St. Peter sandstone), feldspar (Ca-rich albite), calcite (Cyprus chalk), illite (2M1 polytype), and chlorite. We discovered that the specimen of Wyoming montmorillonite (Swy-2) used during Leg 190 contains an unacceptable amount of contamination (mostly by quartz); it was replaced by a relatively pure smectite (Ca-montmorillonite). We also decided to omit kaolinite (Clay Mineral Society Kga-1) from the mixtures because its abundance in Nankai sediments at Site 808 is only 8%-20% of the kaolinite + chlorite clay-sized fraction (Orr, 1992), which amounts to <3% of the typical bulk sediment. Table T1 shows the percentages by dry weight of each mineral in the 14 mixtures that we analyzed.

The standards used for the clay-sized mixtures are smectite (Ca-montmorillonite), illite (Clay Mineral Society Imt-2), chlorite (Clay Mineral Society Cca-2), and quartz (St. Peter sandstone). We included quartz in the mix because of the desire to quantify the nonclay component of the clay-sized fraction in natural specimens and for correcting peak positions relative to quartz (100). Each standard was powdered thoroughly using a Spex Certiprep 5100 mixer mill, suspended in ~500 mL of distilled water with sodium hexametaphosphate dispersant, and disaggregated using an ultrasonic cell disrupter. Particles <2 µm equivalent settling diameter were separated by centrifugation (1000 rpm for 2.4 min; ~320x g). The purity of each clay-sized separate was confirmed by XRD. The average concentration of each suspension was determined by extracting and drying three aliquots at 75°C to obtain dry weight of clay per unit volume of suspension, corrected for weight of dispersant. The weights for smectite probably reflect a hydration state containing two layers of interlayer water. Volumetric proportions of the four components were measured by pipette, then converted to dry weights and weight percentages. Table T2 shows the percentages by dry weight of each mineral in the mixtures that we analyzed. Mixture 7 is nearly pure smectite and was not included in the calculation of normalization factors.

Bulk samples of natural sediment were freeze-dried, hand crushed by mortar and pestle, and powdered for 5 min using a Spex Certiprep 5100 mixer mill. The standard mineral mixtures were also run through the mixer mill for 5 min to improve their homogenization. The bulk powders were then packed gently into XRD sample holders to retain random orientation. The mixtures of standard minerals were analyzed three times each and remixed between each run using the ball mill.

Isolation of clay-sized fractions started with drying and gentle crushing of the mud/mudstone, after which specimens were immersed in 3% H₂O₂ for at least 24 hr to digest organic matter. We then added ~250 mL of sodium hexametaphosphate solution (concentration = 4 g/1000 mL) and inserted beakers into an ultrasonic bath for several minutes to promote disaggregation and deflocculation. This step (and additional soaking) was repeated for highly indurated samples until visual inspection indicated complete disaggregation. Washing consisted of two passes through a centrifuge (8200 rpm for 25 min; ~6000x g), with resuspension in distilled water after each pass. After transferring the suspended sediment to a 60-mL plastic bottle, each sample was resuspended by vigorous shaking and a 2-min application of a sonic cell probe. The clay-sized fractions (<2 µm equivalent settling diameter) then were separated by centrifugation (1000 rpm for 2.4 min; ~320x g). Oriented aggregates of natural samples and standard clay mixtures were prepared using the filter-peel method and 0.45-µm membranes (Moore and Reynolds, 1989). Three separate slides were prepared for each of the standard clay mixtures. The clay aggregates were saturated with ethylene glycol for at least 24 hr prior to XRD analysis, using a closed vapor chamber heated to 60°C in an oven.

The XRD laboratory at the University of Missouri utilizes a Scintag Pad V X-ray diffractometer with CuK radiation (1.54 Å) and a Ni filter. Scans of bulk powders were run at 40 kV and 35 mA over a scanning range of 3° to 35°2 at a rate of 1°2/min and a step size of 0.01°2. Scans of oriented clay aggregates were run at 40 kV and 30 mA over a scanning range of 2° to 23°2, a rate of 1°2/min, and a step size of 0.01°2. Slits were 0.5 mm (divergence) and 0.2 mm (receiving). We processed the digital data using MacDiff software (version 4.2.5) to establish a baseline of intensity, smooth counts, correct peak positions (relative to quartz), and calculate peak intensities and peak areas.

Figure F2 shows the resulting diffractograms for the bulk powder mineral mixtures. Normalization factors were established for the integrated areas of the following peaks: composite clay mineral at ~19.8°2 (d-value = 4.49 Å); quartz (101) at 26.65°2 (d-value = 3.34 Å); a characteristic double peak for plagioclase at 27.77°-28.02°2 (d-value = 3.21-3.18 Å); and calcite (104) at 29.42°2 (d-value = 3.04 Å). We did not record the dimensions of individual clay mineral peaks generated by bulk powders because of low intensities and interference between smectite (001) and chlorite (001) reflections.

Figure F3 shows the resulting diffractograms for the clay-sized mineral mixtures. The normalization factors for clay aggregates are based on the integrated areas of a broad smectite (001) peak centered at around 5.3°2 (d-value = 16.5 Å), the illite (001) peak at 8.93°2 (d-value = 9.9 Å), the chlorite (002) peak at 12.53°2 (d-value = 7.06 Å), and the quartz (100) peak at 20.95°2 (d-value = 4.24 Å).