Data for: A geographic expression of the sepiolite-palygorskite continuum in soils of northwest South Africa
Locations of sepiolite-palygorskite in soils of western South Africa XRD traces of soil clay fractions
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Saturated pastes for all samples were prepared with 300g of <2 mm material to which approximately 100g water was added depending on the clay content of the soil and equilibrated for 24 hours at ambient temperatures, open to gas exchange with the atmosphere. The pH was measured directly in the saturated paste. The water from the saturated paste was extracted under vacuum and the following variables analysed: electrical conductivity (EC) using a calibrated conductivity meter; alkalinity by potentiometric titration to pH 4.5 with 0.01M HCl; major cations (Ca2+, Mg2+, K+, Na+) by atomic absorption spectroscopy; anions (Cl−, SO42−) by ion chromatography; and Si using the blue silicomolybdous acid procedure on the extracted solutions followed by light absorption spectrometry to avoid measuring colloidal silica (Jones and Dreher, 2018 ). Speciation and saturation index calculations were made using PHREEQC Interactive 188.8.131.5227 using the SIT database (SIT.dat) a database implementing the Specific ion Interaction Theory, because it has more data for Mg complexes than the PHREEQ database (PHREEQ.dat). The database SIT.dat is based on version 9b0 from www.thermochimie-tdb.com. The solubility equations and equilibrium constants in the SIT database are: Amorphous silica: SiO2 + 2H2O = H4SiO4 log K = -2.71 (Gunnarsson and Arnórsson, 2000; Rimstidt, 1997); Calcite: CaCO3 = Ca2+ + CO2 log K = -8.48 (Plummer and Busenberg, 1982); Dolomite: CaMg(CO3)2 = Ca2+ + Mg2+ + 2CO32- log K = -17.12 (Robie and Hemingway, 1995); Gypsum: CaSO4.2H2O = Ca2+ + SO42- + 2H2O log K = -4.61 (Garvin et al., 1987); Halite: NaCl = Na+ + Cl- log K = 1.59 (Chase Jr., 1998); Quartz: SiO2 + 2H2O = H4SiO4 log K = -3.74: from Richet et al. (1982). This is more soluble than the log K for quartz in the PHREEQ.dat database where log K = -3.98 (Nordstrom et al., 1990). Sepiolite was absent from SIT.dat and so it was added from Stoessell (1988) to the PHASES block as: Mg2Si3O7.5OH:3H2O + 4H+ + 0.5H2O = 3H4(SiO4) + 2Mg+2; log_k 15.76; delta_h -10.7 kcal, which are the same thermodynamic data as that for sepiolite given in PHREEQ.dat. Al3+ was not measured directly as it is expected to be very low in solution at the pH range encountered in our study (up to pH 9.16) and its direct measurement in solution difficult due to the presence of colloids and ion complexes (Drever, 1997). Al3+ activity was instead calculated using PHREEQ for equilibrium with kaolinite or with illite if kaolinite was absent. These minerals were present either alone or in combination in all of the samples.