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Rectorite-Pyrophyllite-Chlorite-lllite Assemblage in Pelitic Rocks from Fómeque Formation, Central Eastern Cordillera, Colombia Rectorite-Pyrophyllite-Chlorite-lllite Assemblage in Pelitic Rocks from Fómeque Formation, Central Eastern Cordillera, Colombia

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Author(s): Brattli Bjørge

Journal: Geología Colombiana
ISSN 0072-0992

Volume: 20;
Start page: 123;
Date: 1996;
Original page

Keywords: Clay Mineral Analysis | Fómeque Formation | Illite Crystallinity | High Diagenetic-Anchizonal Metamorphism

ABSTRACT
The research described in this paper was performed on samples of slate from the Fómeque formation near Bogotá  in Colombia. Pyrophyllite was found to occur together with mixed layered illite/smectite, chlorite, and illite. Other minerals were quartz, K-feldspar, dolomite and pyrite. Evaluation of the x-ray diffraction patterns reveals that the mixed layered sheet-silicate represents an R1 ordered rectorite with 80-90% illite layers.  The microfabric i.e. the formation of cleavage is developed as a closely spaced cleavage in the phyllosilicate-rich rocks, which grades into a fracture cleavage as the grain size becomes coarser. In the interbedded siltstone no cleavage is observed. It is suggested that the stage of microfrabric developed in these rocks represents high diagenetic to anchizonal conditions.  The illite crystallinity has been measured on glycolated samples from the slate and ranges from 0.47 to 0.55°∆2θ, with a mean of 0.52°∆2θ. Based on the stability of R1 ordered rectorite, the illite crystallinity and the microfabric development, it is proposed that the rocks have been subjected to high diagenetic condition i.e. a temperature of about 200°C at low pressure. At this temperature pyrophyllite can only be stabilized at the expense of kaolinite and quartz if aH20« 1.La investigación aquí descrita se realizó sobre muestras de shale de la Formación Fómeque, cerca a Bogotá, Colombia. Se encontró que la pirofilita ocurre junto con ilita/esmectita, clorita e ilita. Otros minerales incluyen cuarzo, feldespato potásico, dolomita y pirita.  La evaluación de los patrones de difracción de rayos X muestra que el silicato en láminas representa una rectorita de orden R1 con 80-90% de capas de ilita.  La microfábrica i.e. la foliación, se desarrolla como un clivaje estrechamente espaciado en las rocas abundantes en filosilicatos, el cual pasa a a un clivaje de fractura a medida que aumenta el tamaño del grano. En las intercalaciones de Iimolitas no se observa clivaje. Se sugiere que la etapa de microfábrica desarrollada en estas rocas, representa condiciones de diagénesis alta a angimetamórfica.  La cristalinidad de la ilita se ha medido en muestras tratadas con glicol, de pizarra y pasa de 0.47 a0.55°∆2θ, con una media de 0.52°∆2θ. En base a la estabilidad de la rectorita de orden R1, a la cristalinidad de la ilita y al desarrollo de la microfábrica, se postula que las rocas han sido sometidas a condiciones de diagénesis altas i.e. una temperatura de cerca de 200°C a baja presión. A esta temperatura; la pirofilita sólo se estabiliza a expensas de caolinita y cuarzo si aH20 « 1. 
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