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Ammonium-bearing micas in very low-grade metapelites: micro- and nano-texture and composition

Published online by Cambridge University Press:  14 June 2018

Blanca Bauluz  and
Fernando Nieto
Blanca Bauluz*
Affiliation:
IUCA-Facultad de Ciencias, Universidad de Zaragoza, Spain
Fernando Nieto
Affiliation:
Departamento de Mineralogía y Petrología and IACT, Universidad de Granada-CSIC, Spain
*
*E-mail: bauluz@unizar.es
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Abstract

The present study examines the micro- and nano-texture and composition of ammonium-bearing and potassium micas in very-low grade metamorphic black Silurian shales from the SE Iberian Range (NE Spain). Two organic-rich shales were studied by X-ray diffraction (XRD), infrared spectroscopy (IR), transmission electron microscopy (TEM), analytical electron microscopy (AEM) and electron energy loss spectroscopy (EELS). The XRD showed the presence of two populations of micas: pure K micas with d001 = 9.98 Å and ammonium-bearing micas with larger d001 values (10.08 Å and 10.05 Å). The latter values indicate NH4 contents between 13 and 29% in the interlayer, which was confirmed by IR. Interstratifications of smectite and mica layers in the mica packets were not detected by XRD and TEM. Mica packets with sizes ranging from 100 to 250 Å show disordered polytypes and (001) lattice fringes that reflect the presence of K- and NH4-bearing layers (9.9–10.2 Å).

The combination of AEM and EELS analyses on powdered and lamellar samples indicates that micas have typical dioctahedral compositions with highly variable K contents. This variation in K is consistent with the presence of K and NH4 in the interlayers, even though the NH4 and K are not distributed homogeneously; rather they are segregated in nm-sized domains in the mica interlayer.

Keywords

Ammonium-bearing micasEELSTEMvery low-grade metamorphismXRD
Type
Article
Information
Clay Minerals , Volume 53 , Issue 2 , June 2018 , pp. 105 - 116
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Guest Associate Editor: Sebastien Potel

This paper was presented during session GG01-01: ‘Clays in faults and fractures’ during the 2017 International Clay Conference

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