In the Soom Shale, labile soft tissues have been replaced rapidly after death by authigenic clay minerals which now have an illitic composition. There are two possible pathways for this mineralization: (1) initial replacement of soft tissue by kaolinite which was then diagenetically transformed to illite; or (2) direct replacement of soft tissue by illite. Comparison of the compositions of the authigenic illites that replace the soft tissues and of the detrital illites in the host rock show that the former have higher magnesium numbers and enhanced potassium contents. These compositional differences are better explained if illite directly replaced the soft tissues. Kaolinite and/or illite stability are controlled by the pH and the ratio of K+/H+ in the fluid, expressed as: 2(KAl3Si3O10(OH)2) + 3H2O + 2H+ ↔ 3(Al2Si2O5(OH)4) + 2K+. Although the porewaters of the Soom Shale were anoxic and, at times, euxinic, and so might well have favoured kaolinite authigenesis, the conditions within the local environment of the carcasses could have been very different. Ammonia, produced by protein decay, and potassium, concentrated within the tissues, would have been elevated relative to the surrounding pore waters, so that the K+/H+ ratio would have favoured illite authigenesis. Authigenic illites have higher magnesium numbers than detrital sedimentary illites owing to extensive and early pyrite precipitation, indicated by small pyrite framboid diameters (average 4.3 μ), which left the porewaters depleted in iron. The magnesium number in authigenic illites could also have been enhanced by the inclusion of magnesium from seawater.