Fernández Barrenechea, José María

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First Name
José María
Last Name
Fernández Barrenechea
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Geológicas
Mineralogía y Petrología
Cristalografía y Mineralogía
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Now showing 1 - 10 of 13
  • Publication
    Influence of grinding on graphite crystallinity from experimental and natural data: implications for graphite thermometry and sample preparation
    (Mineralogical Society (Great Britain), 2006) Crespo Feo, Elena; Luque del Villar, Francisco Javier; Fernández Barrenechea, José María; Rodas, Magdalena
    This paper examines the effects of shear stress on the structuralparameters that define the ‘crystallinity’ of graphite. The results show that highly crystalline graphite samples ground for up to 120 min do not undergo detectable changes in the three-dimensional arrangement of carbon layers but crystallite sizes (Lc and La) decrease consistently with increasing grinding time. Grinding also involves particle-size diminution that results in lower temperatures for the beginning of combustion and exothermic maxima in the differentialthermalanal ysis curves. These changes in the structuraland thermalcharacteristics of graphite upon grinding must be taken into account when such data are used for geothermometric estimations. Tectonic shear stress also induces reduction of the particle size and the Lc and La values of highly crystalline graphite. Thus, the temperature of formation of graphite according to structural as well as thermaldata is underestimated by up to 100ºC in samples that underwent the most intense shear stress. Therefore, application of graphite geothermometry to fluid-deposited veins where graphite is the only mineralfound should take into consideration the effect of tectonic shearing, or the estimated temperatures must be considered as minimum temperatures of formation only.
  • Publication
    Mechanical graphite transport in fault zones and the formation of graphite veins
    (Mineralogical Society (Great Britain), 2005) Crespo Feo, Elena; Luque del Villar, Francisco Javier; Fernández Barrenechea, José María; Rodas, Magdalena
    This paper describes a vein-shaped graphite occurrence in which, for the first time, the geological, mineralogical and isotopic evidence support its formation by physical remobilization of previously formed syngenetic graphite. The deposit studied is located in the Spanish Central System and it occurs along the contact between a hydrothermal Ag-bearing quartz vein and a graphite-bearing quartzite layer. The characteristics of this occurrence differ from those of fluid-deposited vein-type graphite mineralization in that: (1) graphite flakes are oriented parallel to the vein walls; (2) graphite crystallinity is slightly lower than in the syngenetic precursor (graphite disseminated in the quartzite); and (3) the isotopic signatures of both types of graphite are identical and correspond to biogenic carbon. In addition, the P-T conditions of the hydrothermal Ag-bearing quartz veins in the study area (P <1 kbar, and T up to 360ëC) contrast with the high degree of structural order of graphite in the vein. Therefore, physical remobilization of graphite can be regarded as a suitable alternative mechanism to account for some cases of vein-shaped graphite deposits. Such a mechanism would require a previous concentration of disseminated syngenetic graphite promoted, in this case, by the retrograde solubility of quartz. This process would generate monomineralic graphite aggregates enhancing its lubricant properties and permitting graphite to move in the solid state along distances in the range of up to several metres.
  • Publication
    Caracterización de la materia carbonosa grafitizada de las pizarras silúricas de San Ciprián- Hermisende (Zamora)
    (Sociedad Española de Mineralogía, 2009-09) Crespo Feo, Elena; Rodas, Magdalena; Arche, Alfredo; Fernández Barrenechea, José María; Wada, Hideki; Luque del Villar, Francisco Javier
  • Publication
    Palaeoenvironmental significance of Late Permian palaeosols in the South-Eastern Iberian Ranges, Spain
    (Blackwell Publishing Ltd, 2008) Horra del Barco, Raúl de la; Benito Moreno, María Isabel; López Gómez, José; Arche, Alfredo; Fernández Barrenechea, José María; Luque del Villar, Francisco Javier
    The Late Permian (Wuchiapingian) Alcotas Formation in the SE Iberian Ranges consists of one red alluvial succession where abundant soil profiles developed. Detailed petrographical and sedimentological studies in seven sections of the Alcotas Formation allow six different types of palaeosols, with distinctive characteristics and different palaeogeographical distribution, to be distinguished throughout the South-eastern Iberian Basin. These characteristics are, in turn, related to topographic, climatic and tectonic controls. The vertical distribution of the palaeosols is used to differentiate the formation in three parts from bottom to top showing both drastic and gradual vertical upwards palaeoenvironmental changes in the sections. Reconstruction of palaeoenvironmental conditions based on palaeosols provides evidence for understanding the events that occurred during the Late Permian, some few millions of years before the well-known Permian-Triassic global crisis.
  • Publication
    Late Permian continental sediments in the SE Iberian Ranges, eastern Spain: Petrological and mineralogical characteristics and palaeoenvironmental significance
    (Elsevier, 2005) Benito Moreno, María Isabel; Horra del Barco, Raúl de la; Fernández Barrenechea, José María; López Gómez, José; Rodas, Magdalena; Alonso Azcárate, Jacinto; Arche, Alfredo; Luque del Villar, Francisco Javier
    A detailed mineralogical and petrological study and the analysis of paleosol profiles in continental alluvial sediments of the Late Permian in the SE Iberian Ranges (Spain) allow us to infer the significant environmental changes that occurred during this time period. Three parts have been distinguished in the Late Permian sediments (Alcotas Formation). The lower part includes abundant and well-preserved carbonate paleosol profiles and fine-grained sediments made up by quartz, feldspar, hematite and illite, with scarce kaolinite. The preservation of dolomicrite in some paleosols suggests that they originally developed as dolocretes in an arid to semi-arid climate with marked seasonality. A change towards more humid and acid conditions can be deduced from the presence of siderite and goethite in paleosols in the middle part of the Alcotas Formation. Moreover, the presence of plant remains, coal beds and/or carbonaceous shales at the top of the middle part, and the lack of carbonate paleosols in the upper part of the formation would indicate a further step towards acid conditions. These conditions would increase until the Early Triassic, as indicated by the lack of carbonates and the presence of Sr-rich aluminium phosphate sulphates (APS minerals) at the base of the Triassic (Can˜ izar Formation), which clearly indicates extreme acid conditions during the Permian–Triassic transition of the study area.
  • Publication
    Graphite morphologies from the Borrowdale deposit (NW England, UK): Raman and SIMS data
    (Springer Science Business Media, 2009) Fernández Barrenechea, José María; Luque del Villar, Francisco Javier; Millward, David; Ortega Menor, Lorena; Beyssac, Olivier; Rodas, Magdalena
    Graphite in the Borrowdale (Cumbria, UK) deposit occurs as large masses within mineralized pipe-like bodies, in late graphite–chlorite veins, and disseminated through the volcanic host rocks. This occurrence shows the greatest variety of crystalline graphite morphologies recognized to date from a single deposit. These morphologies described herein include flakes, cryptocrystalline and spherulitic aggregates, and dish-like forms. Colloform textures, displayed by many of the cryptocrystalline aggregates, are reported here for the first time from any graphite deposit worldwide. Textural relationships indicate that spherulitic aggregates and colloform graphite formed earlier than flaky crystals. This sequence of crystallization is in agreement with the precipitation of graphite from fluids with progressively decreasing supersaturation. The structural characterization carried out by means of Raman spectroscopy shows that, with the exception of colloform graphite around silicate grains and pyrite within the host rocks, all graphite morphologies display very high crystallinity. The microscale SIMS study reveals light stable carbon isotope ratios for graphite (δ13C = -34.5 to -30.2%), which are compatible with the assimilation of carbon-bearing metapelites in the Borrowdale Volcanic Group magmas. Within the main mineralized breccia pipelike bodies, the isotopic signatures (with cryptocrystalline graphite being lighter than flaky graphite) are consistent with the composition and evolution of the mineralizing fluids inferred from fluid inclusion data which indicate a progressive loss of CO2. Late graphite–chlorite veins contain isotopically heavier spherulitic graphite than flaky graphite. This agrees with CH4-enriched fluids at this stage of the mineralizing event, resulting in the successive precipitation of isotopically heavier graphite morphologies. The isotopic variations of the different graphite morphologies can be attributed therefore, to changes in the speciation of carbon in the fluids coupled with concomitant changes in the XH2O during precipitation of graphite and associated hydrous minerals (mainly epidote and chlorite).
  • Publication
    Fluid composition and reactions of graphite precipitation in the volcanic-Hosted deposit at Borrowdale (NW England): evidence from fluid inclusions
    (Sociedad Española de Mineralogía, 2008-09) Ortega Menor, Lorena; Luque del Villar, Francisco Javier; Fernández Barrenechea, José María; Millward, David; Beyssac, Olivier; Hizenga, Jan Marten; Rodas, Magdalena
  • Publication
    Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)
    (Mineralogical Society (Great Britain), 2000) Rodas, Magdalena; Luque del Villar, Francisco Javier; Fernández Barrenechea, José María; Fernández Caliani, J.C.; Miras Ruiz, Adolfo; Fernández Rodríguez, Carlos
    Four distinct associations of graphite have been identified in the low-pressure, high-temperature belt of the Sierra de Aracena (SW Spain). Syngenetic occurrences include: (1) stratiform graphite mineralization within a calc-silicate series; (2) disseminated graphite within a terrigenous sequence; and (3) ‘restitic’ graphite within anatectic tonalites and their enclaves. Epigenetic graphite occurs as (4) veins cross-cutting mafic granulites. Graphite in all types of occurrences is highly crystalline, with the c parameter close to 6.70 Å . Such c values correspond to temperatures of formation of ~8008C. The thermal properties of graphite are also typical of well-ordered graphite and provide DTA exothermic maxima ranging from 810 to 8588C depending on the mode of occurrence. The differences among the temperatures of formation estimated by graphite geothermometry, the position of the exothermic maximum in the DTA curves, and petrologic geothermometers are discussed in terms of the applicability of graphite geothermometry to granulite-facies rocks. Carbon isotope analysis yields δ13C values in the range from 31.6 to 21.4% for syngenetic graphite of types I, II and III attributable to biogenically-derived carbon. The heavier signatures for graphite in vein occurrences (δ13C= 17.7 to 18.3%) with respect to syngenetic graphites suggest that isotopically heavy carbonic species were incorporated into the metamorphic fluids (probably as a consequence of decarbonation reactions of the calc-silicate rocks) from which graphite precipitated into the veins. These fluids were strongly channelled through structural pathways.
  • Publication
    Graphite morphologies in the volcanic-hosted deposits at Borrowdale (NW England, UK): preliminary raman and SIMS data
    (Sociedad Española de Mineralogía, 2008-09) Fernández Barrenechea, José María; Luque del Villar, Francisco Javier; Ortega Menor, Lorena; Rodas, Magdalena; Millward, David; Beyssac, Olivier
  • Publication
    Fluid composition and carbon isotope evolution in the Borrowdale graphite deposit (United Kingdom)
    (Elsevier Science Ltd, 2009) Fernández Barrenechea, José María; Luque del Villar, Francisco Javier; Ortega Menor, Lorena; Millward, David; Rodas, Magdalena