Vein antigorites from Elba Island, Italy

Antigorite in veins from Elba Island, Italy, appears as light-green splintery fibers, discontinuously surrounding the massive dark serpentinite, extending along fractures and deformed by compressive shearing. The chemical compositions cluster around Mg2.66Fe0.12Al0.02Si2.00O5(OH)(3.60) with the exact value depending upon the polysomatic structural modulations. Electron diffraction shows superperiodicities variable from 33 to 49 Angstrom and changing from vein to vein, with no relation with the metamorphic grade of the surrounding rocks. Common polysomatic faulting, (001) twinning and parallel association with polygonal serpentine as well as with anomalous (thin wall tubes) chrysotile are the most evident microstructural features. A complete set of X-ray diffraction, IR, NMR and thermal analyses results is presented for a selected number of specimens. With respect to massive antigorite serpentinites, antigorite veins show periodicities shorter than expected; this feature is interpreted as due to shearing stress and/or favourable kynetics able to accelerate the sluggish chrysotile-antigorite transformations.