Nacre-mimicking hybrids of high inorganic content (>50 wt %) tend to show low strain-to-failure. Therefore, we prepared clay nanopaper hybrid composite montmorillonite plate-lets in a continuous matrix of nanofibrillated cellulose (NFC) with the aim of harnessing the intrinsic toughness of fibrillar networks. Hydrocolloid mixtures were used in a filtration approach akin to paper processing. The resulting multilayered structure of the nanopaper was studied by FE-SEM, FTIR, and XRD. Uniaxial stress-strain curves measured in tension and thermal analysis were carried out by DMTA and TGA. In addition, fire retardance and oxygen permeability characteristics were measured. The continuous NFC matrix is a new concept and provides unusual ductility to the nanocomposite, allowing inorganic contents as high as 90% by weight. Clay nanopaper extends the property range of cellulose nanopaper and is of interest in self-extinguishing composites and in oxygen barrier layers. ' INTRODUCTION Polymeric-inorganic hybrid composites are widely used in-dustrially. One example from the automotive industry is melt-processable polymer-clay nanocomposites developed by re-searchers at Toyota. 1 These nanocomposites show improved mechanical, thermal and gas barrier properties at low clay contents, 2 typically below 10 wt %. Reasons include nanoscale dimension of silicate platelets, large aspect ratio, high intrinsic platelet modulus and strength, as well as low thermal expansion. Furthermore, the structure and molecular mobility of the poly-mer matrix itself may be strongly influenced by the nanoscale silicates. 3 By epoxy impregnation of porous clay films 4 or casting from a hydrocolloidal dispersion of a water-soluble polymer and montmorillonite (MTM), 5 composite clay contents exceeding 50 wt % can be obtained. However, already with clay contents exceeding 5 wt %, nanocomposites in bulk form tend to show lowered strength, as exemplified by PA 6-MTM. 2 This is due to the formation of larger scale MTM agglomerates functioning as defects in a brittle material. In nature, nanostructured inorganic-organic hybrid compo-sites form tissues of high mechanical functionality 6 such as bone, antler, enamel, dentin, nacre, sea shells, and egg shells. Nacre in particular has attracted attention because it combines high inorganic content with a favorable combination of modulus, strength, and toughness. 7 The structure is often termed a brick and mortar structure, consisting of thi3n protein adhesive layers bonding microscale calcium carbonate platelets. 8 Nacre has been mimicked in film structures, a maximum of a few μm in thickness, where inorganic platelets are embedded in a polymer matrix. The following studies use preparation techniques providing nano-structural control but are cumbersome and time-consuming. MTM-polyelectrolyte films of up to 5 μm thickness were prepared using layer-by-layer (LbL) deposition techniques. 9,10 In this type of thin films, greatly improved modulus and strength was later reported for MTM-PVA composites, 11 although strain-to-failure was very low. Repeated spin coating of chitosan and monolayer transfer of Al 2 O 3 platelets were used to generate ordered thin hybrid composite materials, 12 which combined 10 GPa modulus with high strength and toughness. Al 2 O 3 -PMMA hybrids with high crack-growth toughness were prepared by ice-templating, freeze-drying, and sintering of ceramic mixtures, followed by monomer impregnation and polymerization. 13 Still, it remains a challenge to prepare large samples of thick (>50 μm) polymer nanocomposites with a high inorganic content (>50 wt %) and yet high toughness in tension. Such materials would be interesting not only in high-technology applications and coat-ings, but also in larger composite structures with mechanical function. It is therefore of interest to consider more facile preparation routes. Recently, a large-scale self-assembly method was used to prepare nacre-mimicking structures based on water-soluble