Person:
Fernández Barrenechea, José María

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First Name
José María
Last Name
Fernández Barrenechea
URI
https://hdl.handle.net/20.500.14352/75219
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Geológicas
Department
Mineralogía y Petrología
Area
Cristalografía y Mineralogía
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1998 - 199912000 - 20062
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    Natural fluid-deposited graphite: mineralogical characteristics and mechanisms of formation
    (American journal of science, 1998) Luque del Villar, Francisco Javier; Pasteris, Jill D.; Wopenka, Brigitte; Rodas, Magdalena; Fernández Barrenechea, José María
    This paper focuses on the similarities and differences between metamorphic graphite (formed in situ from organic matter) and fluid-deposited graphite. We discuss the formation of fluid-deposited graphite in terms of the source of carbon, the characteristics of the C-bearing fluids (the C-O-H system), the mechanisms of carbon mobilization, and the mechanisms of carbon precipitation. New and existing analytical data are compiled on the physical and chemical characteristics of fluid-deposited graphite olitained by the following techniques: optical microscopy, differential ilierrilal analysis, thermogravimetry, X-ray diffraction, Raman spectroscopy, and stable isotope mass spectrometry. Our discussions focus on major, that is, volumetrically significant, worldwide concentrations of graphite
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    Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)
    (Mineralogical magazine, 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.
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    Influence of grinding on graphite crystallinity from experimental and natural data: implications for graphite thermometry and sample preparation
    (Mineralogical magazine, 2006) Crespo Feo, María 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.