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3582376 
Journal Article 
Ubiquitous fibrous antigorite veins from the Lanzo Ultramafic Massif, Internal Western Alps (Italy): characterisation and genetic conditions 
Groppo, C; Compagnoni, R 
2007 
1 
Periodico di Mineralogia
ISSN: 0369-8963 
76 
169-181 
WOS:000255054300010 
Discontinuous veins, I to 20 em thick and from few centimetres to several decimetres long, of fibrous antigorite are widespread in the serpentinites of the Lanzo Ultramafic Massif, Western Alps. The cross fibre antigorite veins consist of rigid and brittle bundles of fibres and typically show a banded structure parallel to the vein selvages. The fibrous antigorite has been characterised by optical microscopy, electron microscopy (SEM-EDS, EMPA and TEM), and vibrational spectroscopy (FTIR and mu-Raman). The determined value of m = 17 (m = number of tetrahedra along an entire wavelength) and thermobarometric data on the associated mafic rocks and rodingites, suggest that fibrous antigorite veins formed during exhumation at T= 350-400 degrees C and P = 0.3-0.6 GPa, i.e. under greenschist facies conditions. The fibrous habit of antigorite may be explained by two different mechanisms, i.e. the crack-seal process and the dissolution-precipitation creep mechanism. Both these processes are compatible with the banded structure of the studied veins and the estimated P-T conditions of formation. 
fibrous antigorite; optical and electron microscopy; spectroscopy; P-T estimates; vein growth