Each clay and soil sample, heated and unheated, was processed as follows: to three different sets of four tubes (15 mL) containing 1.00 g of either soil or clay were added: a) 10.0 mL of KCl 0.10 mol L− 1, b) 400 μL of GPS 5.00 g L− 1 dissolved in KCl 0.10 mol L− 1 plus 9.6 mL of KCl 0.10 mol L− 1, and c) 10.0 mL of saturated GPS solution dissolved in KCl 0.10 mol L− 1; respectively. The tubes were tumbled for 24 h; after which they were spun for 15 min at 2000 rpm; the aqueous phase was discarded and the solid was dried in an oven at 40 °C for 24 h. The FT–IR spectra of the solids were recorded as described below.