An unusual occurrence of palygorskite was found near Montestrutto, lower Val d'Aosta (Italy), within an eclogite-facies leucogneiss of the Sesia-Lanzo zone. Under the optical microscope the mineral appears as a felt of fibres hundreds of μm long and a few μm thick. SEM observation proved each filament to be a bundle of smaller but remarkably long fibres (length mostly 15–20 μm and thickness <1 μm). EDS and TGA showed a water content [zeolitic H2O (≅6%) + structural OH2 (≅8%) + framework OH (≅2%) ≅16 wt.%] lower than average (≅20 wt.%). Structure refinement by the Rietveld method confirmed the clay dioctahedral character and coexistence of two polymorphs, monoclinic [77.4(4)%] and orthorhombic [22.6(6)%], the latter showing anomalous variations of some cell parameters [c shortens to 5.138(3) and a lengthens to 12.903(7)Å]. The high crystallinity of this palygorskite suggests that it formed under low-T hydrothermal conditions due to interaction between Mg-rich hydrous fluids, derived from mantle ultramafics, and Al-rich continental rocks. Predominance of unusually long fibres (>5 μm) causes the Montestrutto palygorskite to be potentially carcinogenic.