Chlorite and illite in some lower Palaeozoic mudstones of mid-Wales

Lower Palaeozoic mudstones from Cwmcadian, Gwynedd are dominantly composed of quartz, illite and chlorite, with subordinate albite, orthoclase and rutile. Chemical formulae are determined for chlorite and illite using a combination of chemical and X-ray data. The formulae indicate that the illite is dioctahedral and suggest that the chlorite structure contains one dioctahedral and one trioctahedral layer. The hypothesis is put forward that both minerals formed from a dioctahedral mineral; illite from a non-expanded form and chlorite from an 'open' form by precipitation of a Fe2+/Mg hydroxide interlayer.