Acicular Mg amphibole in thermal-metamorphic serpentinites from Elba Island, Italy, is affected by polytypic disorder (with the possible coexistence of Pnmn, Pnma, and C2/m structures), chain-width defects, and retrograde hydration reactions. Based on HR-TEM nanostructures, we documented three main hydration stages: (1) formation of chain-multiplicity faults (CMFs); (2) formation of fibrous topotactic talc, elongated parallel to [001]Tlc and resulting from progressive polymerization of CMFs; and (3) pervasive serpentinization. Incipient serpentinization produces isolated inclusions within Mg amphibole, rhombic in shape and parallel to {210}Mg-Am, or elongated parallel to (100)Mg-Am; the inclusions are filled by not-topotactic, curved (001)Srp layers or chrysotile partial fibers. As the Mg amphibole-to-serpentine replacement proceeds, the isolated serpentine inclusions merge together, producing a sponge-like texture, with interconnected fluid paths, from which serpentine can rapidly grow at the expense of enclosing amphibole. We suggest that whereas the first hydration reactions to CMFs and fibrous talc take place at relatively high T and low fluid/rock ratio (fluid circulation confined to microfractures, cleavage planes, and subgrain boundaries), subsequent serpentinization requires significant changes in physical conditions, with definitely lower T and pervasive fluid circulation.

Ferrari, M., Viti, C. (2010). Retrograde hydration sequence in disordered Mg-amphiboles: a TEM investigation. AMERICAN MINERALOGIST, 95(1), 81-91 [10.2138/am.2010.3254].

Retrograde hydration sequence in disordered Mg-amphiboles: a TEM investigation

VITI, C.
2010-01-01

Abstract

Acicular Mg amphibole in thermal-metamorphic serpentinites from Elba Island, Italy, is affected by polytypic disorder (with the possible coexistence of Pnmn, Pnma, and C2/m structures), chain-width defects, and retrograde hydration reactions. Based on HR-TEM nanostructures, we documented three main hydration stages: (1) formation of chain-multiplicity faults (CMFs); (2) formation of fibrous topotactic talc, elongated parallel to [001]Tlc and resulting from progressive polymerization of CMFs; and (3) pervasive serpentinization. Incipient serpentinization produces isolated inclusions within Mg amphibole, rhombic in shape and parallel to {210}Mg-Am, or elongated parallel to (100)Mg-Am; the inclusions are filled by not-topotactic, curved (001)Srp layers or chrysotile partial fibers. As the Mg amphibole-to-serpentine replacement proceeds, the isolated serpentine inclusions merge together, producing a sponge-like texture, with interconnected fluid paths, from which serpentine can rapidly grow at the expense of enclosing amphibole. We suggest that whereas the first hydration reactions to CMFs and fibrous talc take place at relatively high T and low fluid/rock ratio (fluid circulation confined to microfractures, cleavage planes, and subgrain boundaries), subsequent serpentinization requires significant changes in physical conditions, with definitely lower T and pervasive fluid circulation.
2010
Ferrari, M., Viti, C. (2010). Retrograde hydration sequence in disordered Mg-amphiboles: a TEM investigation. AMERICAN MINERALOGIST, 95(1), 81-91 [10.2138/am.2010.3254].
File in questo prodotto:
File Dimensione Formato  
FerrariViti2010AmMin.pdf

non disponibili

Tipologia: PDF editoriale
Licenza: NON PUBBLICO - Accesso privato/ristretto
Dimensione 1.83 MB
Formato Adobe PDF
1.83 MB Adobe PDF   Visualizza/Apri   Richiedi una copia

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11365/22675