Controlled-Humidity XRD Analyses: Application to the Study of Smectite Expansion/Contraction

Smectites are clay minerals that expand and contract dramatically in response to changes in water vapor pressure. The expansion/contraction behavior of smectite has considerable importance in engineering applications (such as the stability of building foundations and the integrity of clay barriers) and affects hydrologic properties. To understand this behavior we have analyzed several purified, homoionic smectite standards over a wide range of relative humidities using X-ray powder diffraction (XRD) and a new computer-automated humidity-control system. Humidity was controlled by mixing dry N 2 with H 2 O-saturated (“wet”) N 2 produced with a gas-washing bubbler held in a constant-temperature bath set at 35°C. Mixing of wet and dry N 2 was controlled with automated MKS mass-flow controllers.