A clay resembling petroleum jelly occurs in an open pit mine, as coatings along the walls of the fracture zones and as small patches along the fissures or as pseudomorphs after spodumene crystals in the pegmatites. Field evidence indicates that the clay is an alteration product associated with spodumene and other lithium-rich aluminum silicates in the pegmatites. The fully hydrated clay has large basal d values. It dehydrates within hours in the laboratory atmosphere through transitional phases into a stable phase, the first basal reflection of which occurs at l3 Å when the relative humidity is under 60 percent. This stable phase has a C-centered cell with considerable stacking disorder and has approximately five cations in the octahedral positions, indicating solid solution between dioctahedral and trioctahedral composition. The approximate structural formula for this stable phase is (Si7.66Al0.34)(Al1.87Fe3+0.15Fe2+0.09Mg1.31Li1.76)O20(F0.65OH3.35)·Ca0.23Mg0.05Na0.11Li0.72K0.04. The data obtained from X-ray diffraction, differential thermal, and infrared analyses are comparable with those for smectites. The stable phase of this clay differs, however, from the known smectites by high lithium content and high degree of crystallinity. The presence of a moderate quantity of Al, and of even a relatively small amount of Mg, rules out the possibility of hectorite. The stable phase of this clay has been named swinefordite in honor of Dr. Ada Swineford.