Illite and illite-smectite, with various morphologies, have been formed on quartz grains by precipitation from solution following dissolution of amorphous Al/Si gels at 150-350 degrees C and 0.75-1.0 kbar (75-100 MPa). Initial solutions include deionized water, chloride and oxalate. The experimental results have application to the formation of authigenic illite in sandstones, from pore waters containing carboxylic acid anions. A series of experiments monitored the effects of reaction temperature and time on the composition and morphology of illitic minerals precipitated following the dissolution of an amorphous gel of illite composition in deionized water. A progressive increase of 70 to 100% in the illite content of illite-smectite occurs between 150 and 350 degrees C, but there is no change in illite-smectite composition for reaction times of 24 hours to 37 days. These results indicate that illite and illite-rich illite-smectite form by direct precipitation from solution and do not require a smectitic precursor. The compositional variation in illite-smectite and illite has little or no effect on clay morphology; all experiments produced a platy-boxwork morphology. Experiments performed at 250 degrees C and initially containing oxalate, in both acidic and alkaline K-bearing solutions, strongly promote illite precipitation following the dissolution of an amorphous Al/Si oxide starting material. The very rapid precipitation that occurs under these conditions is associated with the formation of fibrous/lath-type illites rather than platy types, which are characteristic of slower precipitation in the absence of oxalate. Precipitation of fibrous illite is related to the breakdown of oxalate, which lowers illite solubility by either pH variation or by breakdown of an Al-oxalate complex. Oxalate is not a direct requirement for fibrous illite formation, although it controls fluid composition which favors fibrous illite formation. The association of illite precipitation with carboxylate anion breakdown suggests that the formation of fibrous illite cements in petroleum reservoirs is related to fluids released from source rocks, which contain both aqueous organic acids and inorganic solutes.