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 mm long and a few mm thick. SEM observation proved each filament to be a bundle of smaller but remarkably long fibres (length mostly 15-20 mm and thickness <1 mm). 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) A]. 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 mm) causes the Montestrutto palygorskite to be potentially carcinogenic.
An unusual occurrence of palygorskite from Montestrutto, Sesia-Lanzo zone, internal Western Alps (Italy)
GIUSTETTO, Roberto;COMPAGNONI, Roberto
2011-01-01
Abstract
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 mm long and a few mm thick. SEM observation proved each filament to be a bundle of smaller but remarkably long fibres (length mostly 15-20 mm and thickness <1 mm). 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) A]. 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 mm) causes the Montestrutto palygorskite to be potentially carcinogenic.
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