One goal of the Bureau of Mines is to conserve the nation's mineral resources by developing improved performance materials. Consistent with this goal, the Bureau conducted studies on the formation of undesirable cristobalite in seven different fire clays used as refractories. The studies employed scanning electron micrography, chemical and x-ray analyses, and pyrometric cone equivalent determinations to examine the sample clays and determine the effect of thermal history and stabilizing additives on the amount of cristobalite in various calcines. This research has indicated that as fire clay calcining temperatures increased from 1,390 deg to 1,490 deg c there was a gradual decrease in the percent of cristobalite present in the calcines and a progressive development of mullite crystallization. No cristobalite was found in fire clays calcined above 1,550 deg c, but cristobalite reappeared when the calcines were reheated to temperatures between 1,400 deg and 1,500 deg c. The research also revealed that additions of lithium fluoride (lif) prevented formation of cristobalite in fire clay calcines. The work upon which this report is based was done under an agreement between the Bureau of Mines, U.S. Department of the Interior, and the University of Alabama.