Kaolin and Serpentine Group Minerals

Structural similarities of the kaolin and serpentine minerals bring together in one taxonomic group two classes of minerals with contrasting chemical properties. Halloysite, with two interlayer H20 molecules per formula unit and a 1.0-nm spacing when fully hydrated, is a member of the kaolin subgroup. This chapter is devoted largely to kaolinite and halloysite because of their greater overall importance in soils. Kaolinite, halloysite, and the less common dickite and nacrite are 1:1-layer-structured aluminosilicates of the same ideal composition. The formamide method of differentiating halloysite from kaolinite also is recommended as a means of quantitative determination of kaolinite and halloysite in a mixture. Differential thermal analysis is a useful method of quantitative kaolinite analysis where a satisfactory reference mineral sample is available. Chrysotile, lizardite, and antigorite are the most widely recognized serpentine minerals. Serpentine minerals are unstable in the weathering environment and tend to alter to other minerals during soil formation.