Oxidation of water to hydrogen peroxide at the rock–water interface due to stress-activated electric currents in rocks

Abstract

Common igneous and high-grade metamorphic rocks contain dormant defects, which release electronic charge carriers when stressed. Rocks thereby behave like a battery. The charge carriers of interest are defect electrons h^•, e.g. electronic states associated with O⁻ in a matrix of O²⁻. Known as “positive holes” or pholes for short, the h^• travel along stress gradients over distances on the order of meters in the laboratory and kilometers in the field. At rock–water interfaces the h^• turn into •O radicals, e.g. highly reactive oxygen species, which oxidize H₂O to H₂O₂. For every two h^• charge carriers one H₂O₂ molecule is formed. In the laboratory the battery circuit is closed by running a Cu wire from the stressed to the unstressed rock. In the field closure of the circuit may be provided through the electrolytical conductivity of water. The discovery of h^• charge carriers, their stress-activation, and their effect on Earth's surface environment may help better understand the oxidation of the early Earth and the evolution of early life.

Introduction

Though most oxygen anions in oxide/silicate minerals occur in their common 2− valence state, some can exist in the more oxidized valence 1−, as O⁻. In this paper we present evidence that oxygen anions in the valence state 1− in minerals, which make up the bulk of igneous and high-grade metamorphic rocks, give rise to some unexpected phenomena. The O⁻ are normally dormant, forming inconspicuous and electrically inactive point defects. However, they can be “awakened” when rocks are subjected to mechanical stress. Once activated, the electronic state associated with O⁻ can produce an electric current, which was previously unknown.

First we discuss how O⁻ are introduced into the mineral matrix, why they exist in a dormant form, and how they are become activated when mechanical stresses are applied. Second, we demonstrate that stress-activated electronic charge carriers can flow out of the stressed rock volume and propagate through unstressed rock. When they cross a rock–water interface, they oxidize H₂O to H₂O₂.

It has been shown some time ago that hydroxyl pairs in MgO, introduced into the MgO matrix during crystallization from an H₂O-laden melt or recrystallization in an H₂O-laden environment, undergo a previously unknown redox reaction: they split off H₂ and form peroxy anions, O₂²⁻ (Martens et al., 1976, Freund and Wengeler, 1982):

OH⁻⊕⁻HO ⬄ H₂ + ⊕O₂²⁻

where ⊕ stands for an Mg²⁺ vacancy site. In the peroxy anion, O₂²⁻, the two O⁻ are tightly bonded together, forming a self-trapped, localized, electrically inactive point defect.

Evidence suggests that the same type of redox reaction also occurs in fused silica, where O₃SiOH pairs are converted into H₂ plus peroxy links O₃SiOOSiO₃ (Freund and Masuda, 1991, Ricci et al., 2001). Likewise nominally anhydrous silicate minerals (Freund, 1987, Freund, 2003) are known to always dissolve small amounts of H₂O when crystallizing in H₂O-laden magmas or H₂O-laden high temperature environments (Wilkins and Sabine, 1973, Rossman, 1996, Koga et al., 2003). The solute H₂O occurs in the form of O₃(X,Y)OH, where X, Y = Si⁴⁺, Al³⁺ etc. Hydroxyl pairs are probably the most abundant, but they seem to undergo the same redox conversion, splitting off H₂ and forming peroxy links (Freund, 2003):

O₃XOH HOYO₃ ⬄ O₃X/^OO\YO₃ + H₂

Because crustal rocks consist mostly of nominally anhydrous minerals, which invariably contain small amounts of dissolved H₂O, peroxy links may be very ubiquitous.

From a semiconductor viewpoint, an O⁻ in a matrix of O²⁻ represents a defect electron or hole, also known as positive hole (Griscom, 1990) or phole for short, symbolized by h^•. Accordingly, a peroxy link represents a positive hole pair, PHP (Freund et al., 2006), in which two O⁻ are self-trapped, immobilized and, hence, electrically inactive. PHPs break apart when rocks are subjected to deviatoric stresses, e.g. to stresses that exceed the elastic limit causing dislocations to be mobilized and new dislocations to be generated (Freund et al., 2006, Takeuchi et al., 2006). As dislocations move through the mineral grains, they intersect a PHP causing the peroxy bond to break:

peroxy link + O²⁻ ⬄ loosely bound e' + O⁻ = h^• O₃X/^OO\YO₃ + [XO₄]ⁿ⁻ ⬄ O₃X/^O•_O/YO₃ + [XO₄]^{(n − 1)−}

Eq. (3) describes a process, by which an electron e' is transferred from a neighboring O²⁻ anion (here represented by [XO₄]ⁿ⁻) onto the broken peroxy bond, which captures the electron e'. The donor O²⁻ turns into O⁻ (here [XO₄]^{(n − 1)−}), e.g. a defect electron or phole h^•. In other words, Eq. (3) describes the generation of an electron–hole pair.

In semiconductors like silicon electron–hole pairs recombine very rapidly, often within nanoseconds (Sze, 1981, Van Zeghbroeck, 1997). In the case of the e'–h^• pairs generated by the break-up of peroxy links in silicate matrices, the broken peroxy bond that has captured electron appears to relax in such a way as to delay recombination. Thus, e'–h^• pairs formed in silicates and, hence, in rocks are long-lived (Freund et al., 2006, Freund and Sornette, 2007).

The h^• charge carriers in the silicates are associated with O 2sp energy levels at the upper edge of their valence bands (Canney et al., 1999). When mineral grains are in physical contact, the valence bands are electrically connected. Though their energies will shift from grain to grain, all valence bands form an energy continuum along which h^• charge carriers can propagate. The mode of propagation probably involves a phonon-assisted electron transfer, whereby h^• hop from O²⁻ to O²⁻, maximally at the frequency of thermally activated lattice phonons, ~ 10¹² Hz (Shluger et al., 1992). The O⁻ turn insulating silicate minerals into p-type semiconductors. From a chemistry perspective, an O⁻ is a highly oxidizing •O radical.

Hurovitz et al. described experiments, where freshly ground basaltic minerals immersed in aqueous solution produced detectable amounts of H₂O₂ (Hurowitz et al., 2007). The H₂O₂ production is thought to occur when water reacts with unspecified defects formed at the mineral surfaces during crushing. Possibly h^• charge carriers, activated by the high mechanical stresses levels during crushing, contribute to the oxidizing properties of freshly crushed rock surfaces.

In the experiments as described here, where only a subvolume of a large piece of rock is subjected to increased levels of mechanical stress, h^• charge carriers flow from the stressed into the unstressed rock volume. The stressed rock volume acts as the anode in a battery from where an electric current flows out (Freund et al., 2006). The unstressed rock acts as the electrolyte, through which the h^• flow to reach the Cu contact at the unstressed end of the rock as depicted in Fig. 1a. This Cu contact acts as the cathode.

The electrons e', co-activated according to Eq. (3), cannot spread from the stressed rock into the unstressed rock. However, they can pass from the stressed rock into a metal contact, in our case the steel pistons, and then proceed through a Cu wire to reach the same Cu contact attached to the far end of the unstressed rock. The Cu wire thereby closes the battery circuit. At the interface between the Cu contact and the unstressed rock the e' “shake hands” with h^• and recombine. Knowing that h^• = O⁻, we can write:

O⁻ + e⁻ ==> O²⁻

In the course of this study we addressed three questions:

• Can the h^• charge carriers flow not only through solid rock as depicted in Fig. 1a but also through water as depicted in Fig. 1b?

• What happens at the rock–water interface?

• How can the rock battery circuit be closed, if no wire is available to connect the stressed rock volume with the unstressed rock volume?