Radiolytic Dissolution of Calcite under Gamma and Helium Ion Irradiation

Abstract

Samples of calcite have been irradiated using both the 60.7 MeV helium ion beam of the ARRONAX (Nantes, France) cyclotron and 661.7 keV gamma radiations. The irradiation of calcite pellets results mainly in radiolytically generated gas (H2, CO2). The presence of superficial water during the calcite irradiation in a humid atmosphere is a key factor for the production of radiolysis gases. Besides, the irradiation of calcite–water biphasic media was performed. The gas released after these irradiations is quantified. No significant amount of CO2 has been detected, whereas carbonate ions act as OH• scavengers, thus inducing an increase of H2 production. The amount of hydrogen peroxide created confirms this scavenging mechanism. Finally, ionic chromatography experiments with irradiated solutions allow us to quantify the organic anions (formate HCOO–, acetate CH3COO–, propionate CH3-CH2COO–, and oxalate C2O4 2–) formed by calcite and/or carbonate ion radiolysis and study the variation of calcium carbonate solubility under irradiation. Formate and acetate ions are formed at low irradiation doses, whereas oxalate is favored at high irradiation doses. The increase of calcium ions in solution during the irradiation of calcite–water media indicates that calcite is progressively dissolved during the irradiation. The main difference between calcite–water media and carbonate ion solutions is that calcite acts as a carbonate ion supply in the radiolysis mechanisms.

Carboxylate anion generation in aqueous solution from carbonate radiolysis, a potential route for abiotic organic acid synthesis on Earth and beyond

“…The radiation chemical behavior of formate has already been the subject of several studies (Albarrán et al 1987, Cai et al 2001, Costagliola et al 2017, Draganić et al 1991. The main formate (or formic acid) degradation reaction by water radiolysis products are the following:…”

“…Recently, we have discussed the impact of ionizing radiations (α-beam and γ-Ray) on calcite and calcium carbonate solutions (Costagliola et al 2017). We have shown that calcite dissolution is accompanied by organic anions formation such as formate, acetate and oxalate ions.…”

“…Carboxylate anions (formate HCOO -, oxalate C 2 O 4 2-, glyoxylate (HO) 2 CHCO 2 -, glycolate C 2 H 3 O 3 and acetate CH 3 COO -) are the main organic products during carbonate-bearing aqueous solutions radiolysis (Cai et al 2001, Costagliola et al 2017, Draganić et al 1991. Although the radiation chemistry of carbonate-bearing solutions has been extensively studied within the framework of nuclear waste geological disposal, there is a still major uncertainty regarding: i) the reaction mechanisms, ii) the respective yield of each carboxylate species, and iii) the Linear Energy Transfer (LET) effect (i.e.…”

The Grayness of the Origin of Life

“…Particularly notable is the work carried out by Huber and Wächtershäuser [60], showing that several iron sulfur minerals are able to spontaneously catalyze carbon fixation reactions, producing prebiotic molecules. Non-metals may also drive production of simple organic molecules as recent studies of radiolysis reactions driven by U, Th and K decay demonstrate production of simple organic acids such as acetate, formate and oxalate in addition to the more well known H 2 production [22,61]. It is believed that the earliest forms of proto-life may have had metabolisms catalyzed by environmentally available metal-bearing nanominerals [8].…”

The contribution of water radiolysis to marine sedimentary life

“…Radiolytic yields in pure water are well constrained 5 , 6 and some monominerals (pyrite, various oxides, mordenite, calcite) are known to amplify water-radiolytic H 2 yields when irradiated by γ rays 7 – 9 . Similarly, other oxides and calcite enhance water-radiolytic H 2 production following exposure to α particles 9 – 11 . The effect of mineralogically complex natural materials on H 2 yields is previously unexplored.…”