The Pyrophyllite–Talc Group

This chapter reviews the structure and occurrence of members of the pyrophyllite-talc group in the soil environment, and provides data on the physical and chemical properties that they impart to the soil system. Of all 2:1 phyllosilicates, the members of the pyrophyllite-talc group are the closest to ideal in both octahedral and tetrahedral composition with an empty interlayer position. The pyrophyllite-talc group acts as an ideal case for determination of the effects of octahedral substitution on the thermal stability of 2: 1 phyllosilicates. The end member octahedral composition combined with the pure Si tetrahedral composition of the pyrophyllite-talc group make thermal analysis data worthy of inspection. A general method for recognizing pyrophyllite-talc group minerals in soils by x-ray diffraction (XRD) is to look for peaks in the region of 0.92 to 1.0 nm. The XRD peaks should be unaffected by cation and organic saturation, or by heat treatment.