Kinetic barriers inhibit quartz nucleation and growth at lower temperatures (<50°C [<122°F]). Thus, under ordinary geothermal gradients, the formation of authigenic quartz in fine-grained systems is preceded inevitably by the early stages of compaction. Nucleation sites for quartz precipitation and the abundance and sizes of pores into which quartz cement can be emplaced are limited by the compactional state at the time of precipitation. The two main types of grain alteration that are proposed to yield authigenic quartz, dissolution of biogenic opal and illitization of smectite, occur in different temperature ranges and contrasting compactional regimes. This chapter summarizes petrographic...