During the earliest stages of thermal metamorphism on parent asteroids, primary feldspathic chondrule glass in primitive chondrites began to devitrify to form fine-grained clinopyroxene and crystalline feldspar (Figure 11.1). This resulted in a significant increase in the thermoluminescence (TL) sensitivity of the whole rocks. In some cases, the reaction leading to the crystallization of feldspar may have been catalyzed by water, consistent with the evidence for aqueous alteration in the most primitive ordinary and carbonaceous chondrites.