Por favor, use este identificador para citar o enlazar a este item:
http://hdl.handle.net/10261/28845
COMPARTIR / EXPORTAR:
SHARE BASE | |
Visualizar otros formatos: MARC | Dublin Core | RDF | ORE | MODS | METS | DIDL | DATACITE | |
Título: | Characterization of coexisting NH4- and K-micas in very low-grade metapelites |
Autor: | Nieto, F. | Fecha de publicación: | feb-2002 | Editor: | Mineralogical Society of America | Citación: | American Mineralogist 87(2-3): 205-216 (2002) | Resumen: | Organic-rich Carboniferous shales with associated coal seams, from the Bacia Carbonífera do Douro-Beira (north Portugal), have been studied by TEM as well as by a variety of other methods. NH4 and K micas and berthierine form small subparallel packets of a few layers separated by low-angle boundaries. One- and two-layer ordered polytypes, with some spot enlargement typical of minor disorder, occur in the NH4 micas (i.e., metamorphism tobelites). They exhibit all the characteristics commonly described for sub-greenschist-facies, including a lack of textural and chemical equilibrium. The compositions of the 2:1 layers vary considerably for both K- and NH4-micas, and except for Ti, exhibit similar compositional ranges. The most significant compositional variations are explained by phengitic substitution (Si from 3 to 3.25, Fe + Mg from 0.1 to 0.3), but no evidence of an illitic substitution has been found. NH4 in tobelites was determined by analysis of NH3 using Nessler’s reagent, basal spacing, and 1–(K + Na). The resulting values indicate NH4 contents ranging from 30 to 59% of the interlayer site occupancy. At very low temperatures of metamorphism (e.g., North Sea), the intergrowths of NH4 and K in micas is on the nanometer-scale. With an increase in temperature (e.g., Pennsylvania), NH4- and K-micas are present as different minerals. The Douro-Beira samples represent a still higher-temperature case and, therefore, NH4 and K can be present in the same interlayer sheet but with one cation being dominant. NH4- and K-dominated micas have segregated into well-separated packets with scarce intergrowths and almost no mixed-layers. Hence they show a solvus relationship. | Descripción: | 12 páginas, 10 figuras, 4 tablas. | Versión del editor: | http://ammin.geoscienceworld.org/cgi/content/abstract/87/2-3/205 | URI: | http://hdl.handle.net/10261/28845 | ISSN: | 0003-004X |
Aparece en las colecciones: | (IACT) Artículos |
Ficheros en este ítem:
Fichero | Descripción | Tamaño | Formato | |
---|---|---|---|---|
15087426.pdf | 397,15 kB | Adobe PDF | Visualizar/Abrir |
CORE Recommender
Page view(s)
360
checked on 27-mar-2024
Download(s)
377
checked on 27-mar-2024
Google ScholarTM
Check
NOTA: Los ítems de Digital.CSIC están protegidos por copyright, con todos los derechos reservados, a menos que se indique lo contrario.