Hectorites are natural minerals of the mica family, within the manifold of clay rocks. Organometallic complexes can be inserted within their lamellar structure with interesting catalytic and stereochemical consequences ( Z ). Laponites are synthetic hectorites. They belong to the smectite, i.e., swelling family of clays. Their structure consists of an octahedral layer sandwiched between two tetrahedral layers. In the octahedral layer, all the sites are occupied either by a divalent magnesium ion (trioctahedral clay) or by a lithium ion (one octahedral site in eight). In the tetrahedral layers, there is occasional substitution of a silanol O$iOH group by the fluorinated counterpart O$iF. The counterions responsible for electroneutrality are mostly sodium ions. Laponites exist as platelets about 1000 A in length and about 10 A thick. Their cation-exchange capacity is 50100 meq/ 100 g, about half that of a montmorillonite clay. We have studied laponite B (Laporte Inorganics, Widnes, Cheshire, United Kingdom), whose BET surface area is 330 m*/g and whose cation-exchange capacity is 85 meq / 100 g. Its composition is (percentage per 100 g of clay) SiOz ,55.08; A1203, 0.05; Fe203, 0.06; CaO, 0.20; MgO, 24.82; Na20, 2.55; K20, 0.05; Li20, 1.74; F, 5.6. The interest of laponite for NMR study is twofold: it possesses a rather simple structure, essentially a magnesiosilicate, with an extremely low level of paramagnetic iron impurities, and the water suspensions form gels at high ionic content, due to the binding of the negative silicate planar sheet to the positively charged edges (“house of cards” structure). We have obtained (Bruker AM 300WB and 400 spectrometers) 7Li and 23Na spectra. Longitudinal relaxation times were measured with the inversion-recovery sequence 180”--t-90” followed by a nonlinear regression on the exponential curve, using between 13 and 16 data points. Standard deconvolution procedures were applied to all lineshapes for these Z = 3 nuclei. Theoretical treatments have been provided by Hubbard (2) and Bull (3). Samples consisted of 75 mg of laponite B exchanged with lithium or sodium ions suspended in 4.5 ml of D20. In heavy water, lithium-7 relaxation is quadrupolar (4, 5). The ionic strength was increased by adding the same amount of exchanged clay to aqueous solutions of LiCl or NaCl. The suspension was stirred overnight. Since lithium ions in laponites are either structural or exchangeable, we checked that the high-resolution NMR is opaque to the former and monitors only the latter: