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Licensed Unlicensed Requires Authentication Published by De Gruyter April 29, 2020

New insights into the nature of glauconite

  • Adrián López-Quirós ORCID logo EMAIL logo , Antonio Sánchez-Navas , Fernando Nieto and Carlota Escutia
From the journal American Mineralogist


Glauconite must be assessed as mica-rich mica-smectite R3 interstratified mineral, with the pure end-member mica also having intrinsic K-deficient chemical characteristics (K+ ~ 0.8 apfu). This assertion is in accordance with our X‑ray diffraction (XRD) and high-resolution tranmission electron microscopy (HRTEM) studies and chemical analyses by electron probe microanalysis (EPMA) of mature glauconites in Cenozoic Antarctic sediments that indicate that: (1) It consists of a glauconite-smectite (R3 ordered) mixed-layer silicate, composed mainly of mica-type layers (>90%), but displaying slightly different proportions of Fe(III)-smectite layers (<10%). (2) More mature glaucony grains are characterized by major K+ and VIFe2+ (mica layers) and minor VIFe3+ (smectite layers) content in the interstratified glauconite-smectite. (3) Potassium is stabilized at the interlayer site by the octahedrally coordinated Fe2+. (4) Microtexture of the glauconite crystals are comparable with those of other micas and illite minerals, with straight, defect-free lattice fringes of ~10 Å spacings glauconite packets characteristic of mica with minor interstratified poorly crystalline smectite layers. In addition, our new findings give insights into the glauconitization process and at the same time investigate the potassium-deficient character of the dioctahedral mica “glauconite.” These findings show that glauconite crystallizes by a layer-growth mechanism at the expense of a poorly crystalline smectite precursor and that smectite-to-glauconite transformations are accompanied by a gradually higher octahedral charge deficiency (Fe2+/Fe3+) stabilized by K+ uptake into the interlayer sheet.


This research used samples from the Ocean Drilling Program (ODP). We thank the staff onboard ODP Leg 113 and at the Gulf Coast Repository (GCR) for assistance in core handling and shipping. We thank María del Mar Abad, Miguel Angel Hidalgo, and Isabel Nieto of the Scientific Instrumentation Center (CIC) of the University of Granada for their help during analytical work. We also thank Associate Editor Warren Huff and anonymous referees for their constructive suggestions to improve the paper.

  1. Funding

    Funding for this research was provided by the Spanish Ministry of Science and Innovation (grants CTM2014-60451-C2-1-P and CTM2017-89711-C2-1-P) co-funded by the European Union through FEDER funds. The corresponding author acknowledges a FPI fellowship of the Spanish Ministry of Science and Innovation.

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Received: 2019-10-24
Accepted: 2019-12-12
Published Online: 2020-04-29
Published in Print: 2020-05-26

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