We studied the interstitial liquid phase (Liq_int), the crystallinity of solid phase minerals (kaolinite and saponite), the cation exchange capacity (CEC) and the exchangeable bases of peloids prepared with two medicinal mineral waters (MMWs) from the springs of Lanjarón Spa (Granada, Spain). The waters employed were: “Salud V” (chloride-, sodium- and calcium-rich, bicarbonated, previously unpublished) and “El Salado” (chloride- and sodium rich, bicarbonate, calcic and ferruginous). The solid phase was a mixture of kaolin and bentonite (9:1, w:w) and maturation times of the peloids were 1, 3 and 6 months.

The physicochemical properties of the Liq_int of the peloid differed from those of the MMW: pH increased (from 6 to 8, approximately), Fe content decreased dramatically (from 12 to 0.03 mg l^− 1), while potassium and bicarbonates also decreased and concentrations of other ions such as carbonates, sodium, chlorides and, especially, sulphates, increased. The effect of maturation time on Liq_int properties depended on the MMW employed.

Kaolinite crystallinity in the peloid decreased during maturation (Hinckley Index from 0.71 to 0.52), reaching similar values in the peloids from both MMWs after 6 months. On the other hand, the crystal perfection of saponite increased, revealing the existence of a process of crystallochemical aggradation towards mica due to the presence of exchangeable ions in Liq_int, particularly potassium.

There was a change in CEC and the exchangeable bases from the initial mineral material to the solid phase of the peloid. In the initial material the base sequence was Na⁺ > K⁺ ~ Mg^2 + > Ca^2 + while in the mineral phase of most of the peloids it was Ca^2 + > Na⁺ > Mg^2 + > K⁺. The relationship between the exchangeable bases and the Liq_int was demonstrated.

We demonstrate that the peloid controls and modifies the properties of the starting materials: MMW and minerals. Therefore a new system of liq_int - exchangeable phase - mineral phase interrelationships appears.