The hydrothermal reactivity of a smectite saturated with K was studied experimentally at 300~ and 100 bar in (Na,K) chloride solutions (Na/K = 0, 50 and 100, liquid/ solid ratio = 10/1). X-ray diffraction, TEM and microprobe results show: (1) a partial to total dissolution of the initial smectite layers; and (2) tbe crystallization of newly-formed euhedral t-S. Random IS is formed after 7 days, but an ordered mixed-layer IS containing <30% expandable layers formed in the longest runs (112 days). The IS is characterized by non-expandable layers of two distinct types: dehydrated smectite and illite. The Si content is lower in the IS than in the initial smectite, thus creating a charge deficit, mostly compensated by the introduction of Na to the interlayer space, and yielding a silica release to the solution and subsequent crystallization of quartz and cristobalite.