ionic strength 0.1, 0.01 resp. 0.001M NaNO3; N2-atmosphere; PZC 4-5; SSD calculated from [CW88] based on SSA and NaOH consumption: 160 sites/nm^2; SSD based on crystallographic data 19.8 sites/nm^2 (conclusion, that SSA obtained from BET is to small); assumed that the SSD of Al and Si are proportional to the bulk mineral stoichiometry, that is, NSi = NAl = 0.5 NS