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Colloidal origin of microbands in banded iron formations

M.S. Egglseder1,2,

1School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton, 3800 Victoria, Australia
2Current affiliation: Department of Biomaterials, Max-Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, 14476 Potsdam, Germany

A.R. Cruden1,

1School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton, 3800 Victoria, Australia

A.G. Tomkins1,

1School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton, 3800 Victoria, Australia

S.A. Wilson1,3,

1School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton, 3800 Victoria, Australia
3Current affiliation: Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, T6G 2E3 Alberta, Canada

A.D. Langendam1

1School of Earth, Atmosphere and Environment, Monash University, 9 Rainforest Walk, Clayton, 3800 Victoria, Australia

Affiliations  |  Corresponding Author  |  Cite as  |  Funding information

Egglseder, M.S., Cruden, A.R., Tomkins, A.G., Wilson, S.A., Langendam, A.D. (2018) Colloidal origin of microbands in banded iron formations. Geochem. Persp. Let. 6, 43–49.

Monash University

Geochemical Perspectives Letters v6  |  doi: 10.7185/geochemlet.1808
Received 05 July 2017  |  Accepted 13 February 2018  |  Published 11 April 2018
Copyright © The Authors

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0




Figure 1 (a, b) Transmitted light images (TL) of microbands of chert/quartz (white), chert with colloidal hematite (red, brown) and hematite bands (blue grey). (c) BSE image with colloidal hematite ‘floating’ in quartz crystals, partly liberated (red arrows) and free hematite particles at grain boundaries. (d) TL and (e, f) reflected light images (XPL) displaying irregular layers of hematite particles at truncated quartz crystals (PS) and along quartz grain boundaries with lateral transitions into equigranular quartz aggregates (dashed arrows). (g) XPL image showing hematite particles that surround layered hematite forming core/mantle textures. C: core, H: hematite, M: mantle, IC: iron oxide colloids/particles, PS: pressure solution, Q: quartz, V: void.
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Figure 2 (a, b) BSE images of hematite particles along truncated quartz crystals forming dense aggregates (red outlines) with intergranular pores. Encapsulated colloidal hematite in chert and hematite particles in microbands show similar grain sizes and morphologies. IC: iron oxide colloids/particles, Q: quartz, V: void.
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Figure 3 (a–e) BSE images of BIF laminae illustrating the lateral change from porous chert to hematite microbands with irregular boundaries. (a–c) The porosity is (partly) filled with hematite particles encapsulating quartz grains. (d–e) Hematite forms more larger crystals inside these particle agglomerations, as evident by core/mantle structures resulting in new microbands of hematite. C: core, H: hematite, M: mantle, IC: iron oxide colloids/particles, Q: quartz, V: void.
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Figure 4 Conceptual evolution from BIF precursor sediments to BIF microbands. (a1, a2). Schematic Precambrian depositional environment of BIF precursor minerals. The inset shows the aggregation of precipitates/colloidal phases to form larger particles by flocculation in the water column (a1) and/or attachment in the sediment prior to lithification (a2). (b–e) Development of BIF textures after lithification. Insets show the microscale to nanoscale formation of minerals and textures. (b) Quartz crystallisation (cementation) leads to the entrapment of randomly dispersed colloidal phases. (c) Progressive compression leads to quartz dissolution by DPC, liberation of encapsulated particles and residual layer-parallel accumulation. Dissolved silica forms new particle-free quartz layers (i.e. diagenetic quartz). (d) Simultaneously, accumulated particles transform into more stable iron oxides resulting in the common BIF texture of pristine chert, diagenetic quartz and iron oxide microbands (e).
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