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An unusual occurrence of palygorskite from Montestrutto, Sesia-Lanzo zone, internal Western Alps (Italy)

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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 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 alengthens 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.

Keywords: LOW-T HYDROTHERMAL GENESIS; PALYGORSKITE; RIETVELD METHOD; SCANNING ELECTRON MICROSCOPY-ENERGY DISPERSIVE SPECTROMETRY; THERMOGRAVIMETRY; X-RAY POWDER DIFFRACTION; ZEOLITIC H2O

Document Type: Research Article

DOI: http://dx.doi.org/10.1180/claymin.2011.046.3.371

Publication date: September 1, 2011

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minsoc/cm/2011/00000046/00000003/art00003
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