The central parts of karstic caves in the Ain Khamouda area of Central Tunisia are filled mostly with white clays. Initial examination of these clays using X-ray diffraction, DTA and conventional chemical analysis showed that they consisted of a mixture of halloysite and trioctahedral smectite with appreciable amounts of ZnO. More detailed investigations performed with a diffractometer equipped with a linear detector, a conventional transmission electron microscope and a scanning transmission electron microscope demonstrated that: (i) the halloysite was totally devoid of zinc; (ii) two halloysite phases existed which differed in the hydration states; (iii) the small quantity of MgO (0.2per thousand) present in the bulk sample was located within the halloysite tubes; (iv) zinc was distributed within two phases: a swelling clay of the sauconite-type, and a nearly amorphous Zn- hydroxide; (v) the sample also contained minute particles (3 to 10 nm) of pure silver.