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Growing spherulitic calcite grains in saline, hyperalkaline lakes: experimental evaluation of the effects of Mg-clays and organic acids

Mercedes-Martín, R.; Rogerson, M.R.; Brasier, A.T.; Vonhof, H.B.; Prior, T.J.; Fellows, S.M.; Reijmer, J.J.G.; Billing, I.; Pedley, H.M.

Authors

A.T. Brasier

H.B. Vonhof

J.J.G. Reijmer

S.M. Fellows

Dr Mike Rogerson M.Rogerson@hull.ac.uk
Senior Lecturer and Convener of Geochemistry and Geobiology Group

H.M. Pedley

Abstract

The origin of spherical-radial calcite bodies - spherulites - in sublacustrine, hyperalkaline and saline systems is unclear, and therefore their palaeoenvironmental significance as allochems is disputed. Here, we experimentally investigate two hypotheses concerning the origin of spherulites. The first is that spherulites precipitate from solutions super-saturated with respect to magnesium-silicate clays, such as stevensite. The second is that spherulite precipitation happens in the presence of dissolved, organic acid molecules. In both cases, experiments were performed under sterile conditions using large batches of a synthetic and cell-free solution replicating waters found in hyperalkaline, saline lakes (such as Mono Lake, California). Our experimental results show that a highly alkaline and highly saline solution supersaturated with respect to calcite (control solution) will precipitate euhedral to subhedral rhombic and trigonal bladed calcite crystals. The same solution supersaturated with respect to stevensite precipitates sheet-like stevensite crystals rather than a gel, and calcite precipitation is reduced by ~. 50% compared to the control solution, producing a mixture of patchy prismatic subhedral to euhedral, and minor needle-like, calcite crystals. Enhanced magnesium concentration in solution is the likely the cause of decreased volumes of calcite precipitation, as this raised equilibrium ion activity ratio in the solution. On the other hand, when alginic acid was present then the result was widespread development of micron-size calcium carbonate spherulite bodies. With further growth time, but falling supersaturation, these spherules fused into botryoidal-topped crusts made of micron-size fibro-radial calcite crystals. We conclude that the simplest tested mechanism to deposit significant spherical-radial calcite bodies is to begin with a strongly supersaturated solution that contains specific but environmentally-common organic acids. Furthermore, we found that this morphology is not a universal consequence of having organic acids dissolved in the solution, but rather spherulite development requires specific binding behaviour. Finally, we found that the location of calcite precipitation was altered from the air:water interface to the surface of the glassware when organic acids were present, implying that attached calcite precipitates reflect precipitation via metal-organic intermediaries, rather than direct forcing via gas exchange.

Journal Article Type Article
Publication Date Apr 15, 2016
Journal Sedimentary geology
Print ISSN 0037-0738
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 335
Pages 93-102
DOI https://doi.org/10.1016/j.sedgeo.2016.02.008
Keywords Spherulite, Hyperalkaline, Stevensite, Mg-clays, Organic acids, Lacustrine
Publisher URL http://www.sciencedirect.com/science/article/pii/S003707381600052X
Additional Information This article is maintained by: Elsevier; Article Title: Growing spherulitic calcite grains in saline, hyperalkaline lakes: experimental evaluation of the effects of Mg-clays and organic acids; Journal Title: Sedimentary Geology; CrossRef DOI link to publisher maintained version: http://dx.doi.org/10.1016/j.sedgeo.2016.02.008; Content Type: article; Copyright: Copyright © 2016 Elsevier B.V. All rights reserved.

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