Raman anisotropy in serpentine minerals, with a caveat on identification

AbstractThe serpentine minerals lizardite, polyhedral serpentine, chrysotile, antigorite and 15‐sector and 30‐sector polygonal serpentine have been studied by micro‐Raman spectroscopy, using selected samples, that had been previously characterized. The appropriate crystal orientations were determined by optical microscopy of petrographic sections. Oriented spectra, obtained using Nd‐YAG green laser radiation (532 nm), were deconvolved, extracting wavenumber and intensity values for the peaks, possibly overlapping and forming complex bands. Relevant Raman anisotropy is common and relevant in serpentine. Depending upon the orientation of the impinging laser, significant wavenumber shifts occur (up to 10 cm−1, mostly in polyhedral serpentine and lizardite). Furthermore, also, important intensity variations (up to 1 order of magnitude) occur in polyhedral serpentine, lizardite, chrysotile and antigorite as well. On the one hand, the possibly characteristic peaks have been identified and discussed. On the other hand, caution is suggested as far as the micro‐Raman characterization of polyphasic, variably oriented serpentine minerals is concerned

Evidence of extensional metamorphism associated to Cretaceous rifting of the North-Maghrebian passive margin : the Tanger- Ketama Unit (External Rif, northern Morocco)

The distribution pattern of diagenetic conditions to very low-grade metamorphism in the eastern Rif has been determined based on a study of clay-mineral assemblages and illite crystallinity of Mesozoic metapelites. Low-grade conditions were reached in marbles and also in the Beni-Malek serpentinites, as suggested by the mineral assemblages present in the marbles and antigorite growth in serpentinites. Previous thermochronological data are based on i) 40Ar/39Ar in amphiboles from greenschists, ii) K/Ar in white micas from metasandstones, and iii) fission tracks in apatites and zircons from metasandstones. These data indicate a Late Cretaceous age (∼80 Ma) for the very low- to low-grade metamorphism. We propose an evolutionary model for the Tanger-Ketama Unit consisting of a Lower Cretaceous sequence deposited in half-graben basins over an exhumed serpentinized mantle in a setting similar to the West Galician non-volcanic margin. The sediments underwent diagenesis to very low-grade metamorphism under relatively high heat flow in this extensional setting. Miocene contractional deformation of the Tanger-Ketama Unit resulted in a penetrative crenulation cleavage associated to asymmetric inclined folds. This crenulation developed, mostly by solution-transfer processes, without significant mineral growth. Miocene metamorphism reset the apatite fission-tracks, but metamorphic conditions were not high enough to reset either the K/Ar ages or the zircon fission track

The role of H3O in the crystal structure of illite

9 páginas, 5 figuras, 3 tablasIn spite of decades of research on the subject, the crystal structure of illite is still poorly understood. The purpose of this study was to address this problem by investigating the nature of the interlayer content in illite IMt-2 from Silver Hill, Montana, using analytical transmission electron microscopy (ATEM), thermogravimetry (TG), and X-ray powder diffraction (XRPD) analyses. The ATEM data, together with literature and TG results, yielded the formula K0.70Na0.01(H2O)0.42(Al1.53Fe2+0.06Fe3+0.19Mg0.28) =2.06(Si3.44Al0.56)O10(OH)2 or, assuming the presence of H3O+, K0.69Na0.01(H3O)+0.28(Al1.47Fe2+0.06Fe3+0.19Mg0.28) =1.99(Si3.40Al0.60)O10(OH)2. The first formula indicates surplus interlayer and octahedral species, whereas the second shows no excess. The XRPD data were refined by Rietveld techniques, down to an Rp factor of 10.48–13.8%. The mineral composition consists largely of illite-2M1, illite-1M, and minor quartz. Although the refinement accuracy is limited by the intrinsic poor quality diffraction of the illites, the partially refined model is consistent with the chemical composition; in particular, attempts to introduce octahedral cations in excess of 2 were fruitless. All the results support the simple structural model, by which the illite structure strictly corresponds to a dioctahedral mica with H3O+ replacing K. As a consequence, the crystalchemical formula of illites should be calculated on the basis of six tetrahedral plus octahedral cations.Proyecto de investigación GL 2007-66744 y Grupo de Investigación RNM-179 y RNM-325 de la Junta de AndalucíaPeer reviewe

Naturaleza mineralógica de la denominada "oxiclorita" en rocas del Complejo Nevado-Filábride (Cordilleras Béticas)

2 páginas. Resumen de la comunicación presentada en la IX Reunión de la Sociedad Española de Mineralogía (Valladolid, 26-30 junio 1989)Peer reviewe

Oriented, not-topotactic olivine-serpentine replacement in mesh-textured serpentinized peridotites

Partially serpentinized harzburgites from Southern Tuscany, Italy, show serpentine replacing the peridotitic minerals, as rims around olivine and thin lamellae parallel to pyroxene cleavage. Exempt from post-serpentinization tectonometamorphic overprints, these mesh-textured serpentinites offer a favourable setting for the study of seafloor serpentinization. Studied by HRTEM and AEM, the olivine serpentine replacement reveals a complex sequence of reaction steps. Initially, olivine dissolves forming a silicon-enriched amorphous domain, where early serpentine nuclei are formed. These nuclei recrystallize producing oriented columnar lizardite. The lizardite in the rim shows silicon excess, due to intermixed amorphous or talclike layers. No chrysotile fiber occurs at the reaction front. Although the olivine-to-lizardite reaction is clearly not-topotactic, recrystallization of early formed serpentine leads to large lizardite sectors, oriented with (001) almost parallel to the reaction front. As the olivine-to-lizardite reaction is estimated to occur in the upper 300-500°C range, lizardite has to be considered as the high-temperature serpentine phase in retrograde serpentinites

Naturaleza mineralógica de la denominada "oxiclorita" en rocas del Complejo Nevado-Filábride (Cordilleras Béticas)

2 páginas. Resumen de la comunicación presentada en la IX Reunión de la Sociedad Española de Mineralogía (Valladolid, 26-30 junio 1989)Peer reviewe