Experiments were conducted to identify the rock-water and microbial interactions influencing accelerated smectite-clay formation. Packed columns and stirred batch reactors contained Äspö granodiorite, artificial groundwater mimicking that from Äspö and combinations of three types of subsurface chemolithotrophic bacteria, two of which were indigenous to the Äspö rocks. Results showed evidence that, within 5 days under anaerobic reducing conditions, all three of the bacterial types produced copious biofilamentous ‘meshes’ across porespaces, apparently using the larger grains as anchor points. The biofilaments quickly became encrusted with fine grained material and surrounded with neoformed clay-like deposits. In contrast, the abiotic controls showed little or no evidence of clay formation suggesting that this process is biologically induced or controlled. A second series of abiotic experiments to determine the effects of increased acidity showed evidence of mineral pitting and dissolution along with an increase in concentration of soluble species thought to be important in smectite formation (i.e. Si, Al, Mg, Fe, Ca, Na). However, there was no evidence of clay formation, and the biotic experiments showed no signs of bulk scale pH change, suggesting that either the bacteria are actively concentrating relevant chemical species at a local level or they are acting as templates or nucleation points for clay formation.