Experiments have been performed in which cores of clay-free sand, and clay-forming mineral phases placed beneath the cores, have been treated hydrothermally with various solutions. Using experimental conditions in the range 150-500 degrees C and 0.75-1.0 kbar (75-100 MPa) illite, illite-smectite, kaolinite, 7 A-chlorite and trioctahedral smectite have been precipitated in the sand pore space with morphologies similar to those of authigenic clay minerals in sandstone petroleum reservoirs. This study demonstrates that the mechanism, rate of clay precipitation, and controls on clay morphology and mineral stability can all be determined by experimental studies. These results have direct application to the interpretation of clay mineral paragenesis in reservoir sandstones. The study demonstrates: 1) authigenic clays in sandstones are direct precipitates from solution, and do not require a preexisting clay coating; 2) grain surface effects, such as abrasion, and detrital clay coatings do promote nucleation of clays; and 3) clay morphology is controlled by precipitation mechanism, specifically, the formation of metastable clays which are subsequently modified. The synthesis of authigenic-type illite and illite-smectite is by direct precipitation from solution following dissolution of mineral phases such as detrital illite or feldspar. Both platy and fibrous illite morphotypes are synthesized; platy illites are more readily formed. The compositions of synthetic illite and illite-smectite clays are controlled by temperature and a (sub K (super +) ) / a (sub H (super +) ) fluid composition ratio. Platy illites show subtle changes in morphology with compositional variation. Illite-smectite and illite with low K content forms a crenulated boxwork texture, while more K-rich illites approaching muscovite composition form larger, discrete plates. No correlation between illite composition and fibrous formation is recognized. Kaolinite and chlorites were also precipitated in experiments. Vermiform kaolinite appears to be a direct precipitate. Kaolinite morphology, however, is controlled by precipitation mechanism; isolated, discrete kaolinite plates are associated with early formation of a mixed-layered smectitic phase. Chlorite synthesis proceeds via the formation of metastable 7 A-chlorite and expandable phases, which have previously been recognized as possible precursors to 14 A chlorites in reservoir sandstones.