Vermicular kaolinite epitactic on primary phyllosilicates in the weathering profiles of anorthosite

The microtextural changes in the kaolinization of primary phyllosilicates i ncluding biotite, sericite, clinochlore and muscovite were investigated by scanning electron microscopy (SEM) and microchemical analysis of thin secti ons of weathered anorthosite. Kaolinization began at grain edges and propag ated toward the interior. Grains were highly fanned out from the edges and exfoliated into several flakes along the basal cleavages, producing lenticu lar voids. Finally, long vermicular kaolinite pseudomorphs were formed afte r primary phyllosilicates. Statistical analysis showed a ninefold increase in volume during the kaolinization of biotite, suggesting that most Al in t he kaolinite was imported from ambient weathering solution. Weathering prim ary phyllosilicates supplied templates suitable for the thick epitactic ove rgrowth of kaolinite to form long vermicular pseudomorphs. Al was sufficien tly available due to the intense weathering of soluble anorthosite. Althoug h present in small amounts, primary phyllosilicates gave high volumetric an d mineralogical contributions to the weathering profiles by facilitating ka olinite precipitation.