mica group, which includes the common mica minerals and illite, the smectite
group and the chlorite group.
Illite is a term used to describe clay-sized mica-type minerals and is not a spe-
cific mineral name; however, in general, illite composition is similar to muscovite
mica (Fig. 4.11). In the muscovite structure, one of every four tetrahedral silicons
is replaced by aluminium. The regular replacement of tetravalent silicon by triva-
lent aluminium means that the tetrahedral sheet in muscovite carries a strong net
negative charge. Ideally, illite has dioctahedral structure, but some of the octa-
hedral aluminium is substituted by Fe^2 +and Mg^2 +(Box 4.6), resulting in a net
negative charge for the octahedral sheet. In total, the illite 2 : 1 unit has a strong
net negative charge, known as the layer charge. This negative charge is neutral-
ized by large cations, usually K+, which sit between the 2 : 1 units and bond
ionically with basal oxygens of the opposing tetrahedral sheets in six-fold
coordination. The radius ratio rule predicts that K+should exist in eight-fold or
The Chemistry of Continental Solids 89OHOHOHOHOHOHOHOHOctahedral
sheetTetrahedral
sheet1:1 clay
mineral
structure(a)(b)Silicon
Aluminium
OxygenOHOHOHOHOHOHOHOHOH OH OH OHFig. 4.8Schematic diagram to show how the octahedral and tetrahedral sheets, seen as
separate entities in (a), can be merged to form 1 : 1 clay mineral structure shown in (b).