Hydrous micas differ from normal micas in that they contain less potassium and more water. In the calculation of structural formulae from their chemical analyses, two methods have been used which rely on the following different assumptions:1. As these minerals are often fine-grained and may contain adsorbed water, no reliance is placed on the value of H2O+ (water lost above 105° C.) as a measure of structural water. The water content is ignored and the structural formula is calculated on the basis of 20 oxygens plus 4 hydroxyls per structural unit. This method has been used by Hendricks and Ross and by Grim and Bradley.2. The water which is recorded as H2O+ in the chemical analysis is assumed to represent hydroxyl groups in the aluminosilicate layers of the minerals. Thus, instead of having 20 oxygens and 4 hydroxyl ions per structural unit as in normal micas, formulae are derived which show less than 20 oxygens and more than 4 hydroxyls, the total oxygen plus hydroxyl remaining 24. This method has been used by Brammall, Leech, and Bannister.