Methods
Thin-section petrography was performed at samples from the transition of bleached to dark rock sections (profile length: 40 mm), see Fig.
2, SEM P5.
X-ray diffraction patterns were recorded using a PANalytical X’Pert PRO MPD Θ-Θ diffractometer (Cu-Kα radiation generated at 40 kV and 30 mA), equipped with a variable divergence slit (20 mm irradiated length), primary and secondary soller, Scientific X′Celerator detector (active length 0.59°), and a sample changer (sample diameter 28 mm). The samples were investigated from 2° to 85° 2Θ with a step size of 0.0167° 2Θ and a measuring time of 10 s per step. For specimen preparation the top loading technique was used. The chemical composition of powdered samples was determined using a PANalytical Axios and a PW2400 spectrometer. Samples were prepared by mixing with a flux material (Lithiummetaborate Spectroflux, Flux No. 100A, Alfa Aesar) and melting into glass beads. The beads were analyzed by wavelength dispersive X-ray fluorescence spectrometry (WD-XRF). To determine loss on ignition (LOI) 1,000 mg of sample material was heated to 1,030°C for 10 min.
The cation exchange capacity (CEC) was measured using the Cu-Triethylenetetramine method (Meier and Kahr
1999).
For electron optical investigations an environmental electron scanning microscope (ESEM) (Quanta 600F, FEI) operated at low-vacuum mode (0.6 mbar) was used. Therefore, sputtering of the samples with gold or carbon was not necessary. The microscope is equipped with the EDX-system Genesis 4000 of EDAX.
To measure midinfrared (MIR) spectra, the KBr pellet technique (1 mg of sample/200 mg of KBr) was applied. Spectra with a resolution of 2 cm−1 were collected using a Thermo Nicolet Nexus FTIR spectrometer (MIR: beam splitter: KBr, detector DTGS TEC).
Thermoanalytical investigations were performed using a Netzsch 409 PC thermobalance equipped with a DSC/TG sample holder linked to a Pfeiffer Thermostar quadrupole mass spectrometer (MS). 100 mg of powdered material previously equilibrated at 53% relative humidity (RH) was heated from 25 to 1,000°C with a heating rate of 10 K/min.
Total organic carbon (TOC) and sulfide/sulfate sulfur concentrations were measured with a LECO CS-444-Analysator after dissolution of the carbonates. Carbonates had been removed by treating the samples several times at 80°C with HCl until no further gas evolution could be observed. Samples of 170–180 mg of the dried material were used to measure the total carbon (TC) and S-content. TIC was calculated by the difference of TC-TOC. The samples were heated in the device to 1,800–2,000°C in an oxygen atmosphere and the CO
2 was detected by an infrared detector. A depth profile of sample material with four subsamples from the weathered plate (surface to 5 mm, 5–10 mm, 10–15 mm, 15–20 mm = bottom) was analyzed for organic carbon and sulfate/sulfide sulfur concentrations using a C/S analyzer (CS-800, Eltra GmbH, Neuss). Pulverized sample material was taken for C/S analyses. 100 mg of dried (105°C) sample was mixed with 500 mg Fe and 1,000 mg W and subsequently catalytically combusted in an induction furnace in an oxygen atmosphere at about 1,400°C. Carbon and sulfur were detected as CO
2 and SO
2, respectively, using an infrared absorption cell. Appropriate mixtures of quartz sand, CaCO
3, and Na
2SO
4 were taken as calibration and quality control standards that were run after 10 sample measurements. The detection limit for both C and S was 0.01 wt%. Inorganic carbon (IC) was analyzed as the difference between the total carbon (TC) content in a non-treated and a pre-treated (HCl 25%) sample. The samples contained no IC. 100 mg of dried (105°C) sample was combusted without additives in an electric resistance oven in an oxygen atmosphere at 700°C and measured as SO
2 in an infrared absorption cell. Appropriate mixtures of quartz sand and pyrite were used as calibration and quality control standards that were run after 10 sample measurements. The detection limit for
S
sulfide was <0.25 wt%. At the chosen temperature, complete
S
sulfide combustion takes place within a few minutes while
S
sulfate is still stable (Brumsack
1981). Assuming that all sulfur is present either as sulfide or sulfate in the sample, the sulfate content was calculated as the difference between
S
total and
S
sulfide. In each case, three subsamples were analyzed.
Mercury intrusion porosimetry was performed using a
Carlo Erba mercury porosimeter (Porosimeter 2000). Total porosity and pore-size distribution were calculated from the volume of mercury pressed into the pore space as a function of pressure (Modry et al.
1981; van Brakel et al.
1981). The investigations were carried out with pressures up to 2 kbar, which allows the evaluation of pore radii of about 4 nm (small mesopores). At least three specimens of every rock type were analyzed. The results show similar distributions for the respective rock type. Pore radii were calculated assuming cylindrical pore spaces (Ritter and Drake
1945a,
b; Washburn
1921).
Vertical scanning interferometry: Surface topography variations of discolored versus dark-colored surface samples were examined using vertical scanning interferometry (VSI). A
ZeMapper, manufactured by Zemetrics Inc., Tucson, AZ was used. VSI is an optical profilometry method that provides a large field of view of up to several mm
2 and a high vertical resolution (<1 nm). For this study, white light interferometry mode was used. Two Mirau objectives (magnifications: 20× and 100×) were utilized. The specifications of the three-dimensional data sets are given in Table
1. The obtained three-dimensional data sets were used for roughness parameter analysis. According to Fischer and Lüttge (
2007), roughness parameters were analyzed as a function of field-of-view size. 24 data sets of discolored and dark-colored slate were analyzed, respectively.
Table 1
VSI data set parameters
20× | 747 × 747 | 365 |
100× | 149 × 149 | 72 |