Takashi Murakami

This thematic issue of Elements describes the events at the Fukushima Daiichi nuclear power plant on March 11, 2011, and the aftermath.

We have examined a paleosol in Pronto, Canada, which formed at ~2.45 Ga and under very low O₂ conditions, to reconstruct ancient weathering processes. Chlorite, which replaced biotite in parent granite, consists of almost complete chlorite layers as revealed by high-resolution transmission microcopy. Chlorite grains were fragmented gradually with increased intensity of weathering. The fragmented chlorite grains were partly or largely replaced by sericite formed from K-metasomatism, which strongly suggests that almost complete chloritization of biotite preceded sericitization. In the upper part of the paleosol, chlorite-biotite mixed layers formed but the formation of these layers cannot be explained by mere chloritization and sericitization, which are usually considered to occur concurrently during post-weathering events. Our observations and analyses suggest that the chlorite-biotite mixed layers were formed as follows: chlorite, with no or little biotite, was formed from biotite in parent granite prior to weathering. Chlorite was then weathered by dissolution and fragmentation at ~2.45 Ga, and chlorite-vermiculite (or smectite) mixed layers were formed by layer-by-layer transformation in the weathering profile (note that vermiculite is not actually observed in the Pronto paleosol). Following burial of the weathering profile, the vermiculite layers in the chlorite-vermiculite mixed layers were converted to biotite layers, by means of K-metasomatism, to form the chlorite-biotite mixed layers. The chlorite-biotite mixed layers in the Pronto paleosol were therefore chlorite-vermiculite mixed layers at the time of weathering.

Fresh and weathered granite from drill cores in Tono, Gifu, Japan, was examined to understand weathering products and the mechanisms of chlorite and biotite weathering under low PO₂ conditions. A fresh sample from 365 m depth, a slightly weathered light-green sample from 367 m depth, and a nearly fresh sample from 369 m depth (but with brown stains on fractures), were investigated. The XRD, SEM, EMPA, and TEM analysis of green grains present within chlorite, biotite, and plagioclase grains and in veins was found to be Fe²⁺-rich corrensite [about 40 wt% FeO with Fe/(Fe + Mg) = 0.94]. The corrensite is interpreted to have formed from chlorite and biotite via a dissolution-precipitation mechanism. The 0.69. These results indicate that on dissolution of chlorite and biotite, Fe²⁺ was transported as Fe²⁺ and precipitated as Fe²⁺-rich corrensite and a part of the dissolved Fe²⁺ was oxidized to amorphous Fe³⁺ (hydr)oxides under low PO₂ conditions. The formation of Fe²⁺-rich corrensite and that of Fe²⁺-rich smectite or vermiculite in the laboratory at 1 atm of PCO₂ and ≤3 × 10⁻⁵ atm of PO₂ (Murakami et al. 2004) suggest that a possible Fe²⁺-bearing product during Precambrian weathering is Fe²⁺-rich sheet silicates but not siderite.

We investigated dehydration processes in uranyl phosphate minerals of the meta-autunite group that consist of uranyl phosphate sheets and interlayer cations, and water molecules. Meta-autunite [Ca(UO₂PO₄)₂·6H₂O], metasaléeite [Mg(UO₂PO₄)₂·8H₂O], and metatorbernite [Cu(UO₂PO₄)₂·8H₂O] were selected for our study. The change in basal spacing between two adjacent uranyl phosphate sheets with temperature was examined by temperature-controlled X-ray diffraction (XRD) analysis from room temperature (RT) to 300 °C to determine structurally distinct, dehydrated phases. Thermogravimetric and differential thermal analyses (TG/DTA) were also performed under conditions similar to those used for the XRD analysis to clarify the hydration states of the dehydrated phases. Retention of the structure of the uranyl phosphate sheets under a high vacuum, equivalent to 300 °C, was confirmed by transmission electron microscopy. Meta-autunite, metasaléeite, and metatorbernite decreased their basal spacings by losing water molecules. Comparison of the TG/DTA and XRD results indicates that the changes in basal spacings of the dehydrated phases with temperature are as follows: 8.32 Å (6 H₂O per unit formula) at RT, 7.31 (? H₂O) and 6.68 Å (? H₂O) at 75 °C, 6.34 Å (2 H₂O) at 120 °C, and 5.81 Å (1 H₂O) at 300 °C for meta-autunite; 8.29 (8 H₂O) and 7.73 Å (? H₂O) at RT, 6.62 Å (probably, 2 H₂O) at 40 °C, 6.54 Å (2 H₂O) at 160 °C, and 5.52 Å (1 H₂O) at 300 °C for metasaléeite; and 8.61 Å (8 H₂O) at RT, 8.07 Å (4 H₂O) at 100 °C, 6.58 Å (2 H₂O) at 200 °C, and 5.60 Å (1 H₂O) at 300 °C for metatorbernite. The dehydration processes revealed by XRD and TG/DTA under similar experimental conditions are slightly different from those obtained by previous studies. Our results clearly demonstrate the presence of previously unknown dehydrated phases of the meta-autunite group minerals with basal spacings less than 6 Å that may have distinct thermodynamic properties.

Understandings of U transport at the Earth’s surface are quite important especially for the disposal of radioactive waste and the remediation of contaminated ground water. We examined long-term U transport in and around the Koongarra U deposit, Australia, which has been subjected to weathering for the past 2 million years. The formation of saléeite [Mg(UO₂)₂(PO₄)₂·₁₀H₂O] occurred at the rim of apatite [Ca₅(PO₄)₃(F,OH)], where the ground waters had about 200 - 400 ppb U (∼10^-7 mol·l^-1) and were undersaturated with respect to saléeite. The laboratory experiments using apatite and U-containing solutions and Rutherford backscattering spectrometry of the apatite surface have suggested that a leached layer of about 100 nm thick is formed on dissolving apatite, saléeite is precipitated only in the leached layer by local saturation, and saléeite precipitated is added to the interface between the leached layer and solution. At lower U concentrations (∼10^-8 mol·l^-1), U occurred as nanocrystals (20-100 nm in size) of uranyl phosphates such as metatorbernite [Cu(UO₂)₂(PO₄)₂·₈H₂O], scattered between and attached firmly to nanocrystals (2-50 nm in size) of goethite and hematite. A possible mechanism is that U, P, and Mg or Cu adsorbed onto ferrihydrite form nanocrystals of uranyl phosphates during crystallization of goethite and hematite from ferrihydrite. The above two mechanisms significantly lower the U concentration in the ground water that flows into the Koongarra creek, 200 m downstream from the ore deposit, where the U concentration is ∼10^-10 mol·l^-1. We conclude that the two mechanisms of uranyl-phosphate mineralization control the long-term U transport at Koongarra.

Biotite dissolution in the laboratory and in nature was examined and compared to elucidate certain aspects of the weathering processes. Batch dissolution experiments of fresh biotite [(K_0.91Na_0.01)(Mg_0.40Fe_2.07Mn_0.05Al_0.14Ti_0.19)(Si_2.82Al_1.18)O₁₀(OH)₂] in granite were carried out at 150 °C for 1 to 56 days. Examination by scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) revealed that, in the early stage, dissolution proceeds from the edges of crystals inward and secondary minerals such as Fe oxide are precipitated mostly at the edges, with only a few secondary minerals found on the basal surfaces. A dissolution experiment using a mixture of biotite and muscovite, done at 150 °C for 7 days, indicated that hematite crystals formed mostly at the edges of biotite but not on muscovite. This observation suggests that released Fe is precipitated before it diffuses into the bulk solution. Because a dissolution rate at the edge is larger by two orders of magnitude than at the basal surface (Turpault and Trotignon 1994), precipitation before diffusion of dissolved elements to bulk solution well explains the preferential secondary mineralization at the edges in the laboratory experiments. SEM-EDS of fresh to slightly weathered biotites revealed that early stage weathering proceeds in the same way as in the laboratory with the edges being preferentially weathered and secondary minerals being precipitated mostly at the edges. Because of the similarity between the occurrence of secondary minerals in the laboratory experiments and in nature, the laboratory results elucidate the early stage weathering conditions, namely that (1) supersaturation with respect to secondary minerals in a solution occurs around biotite, i.e., the solution is poorly connected to a main flow pathway of water, and (2) once supersaturation is achieved, secondary minerals are precipitated mainly at the edge before some released elements diffuse into the solution. The immobility of water immediately adjacent to primary minerals partly explains the large difference in dissolution rate between the laboratory and natural samples. A phlogopite dissolution experiment was done at 150 °C for 5.5 days to examine the effect of biotite composition on vermiculitization. High-resolution transmission electron microscopy revealed that vermiculite layers are formed by layer-by-layer transformation within phlogopite after 5.5 day dissolution but not within biotite after 56 day dissolution or within slightly weathered biotite. A comparison of our data with results of other studies indicates that the formation of vermiculite layers between biotite layers occurs even in the early stage when biotite is not Fe-rich [more than 0.8 Mg per O₁₀(OH)₂]. When biotite is Fe-rich [less than 0.4 Mg per O₁₀(OH)₂]—like the present one—released Fe is precipitated as, for example, Fe hydroxide, and vermiculite is rarely formed because of limited availability of Mg. Thus, a higher Mg content in biotite facilitates the formation of vermiculite, at least in the early stage. Because vermiculite dissolves at a much slower rate than biotite, Mg-rich biotite dissolves at a slower rate than Fe-rich biotite. In the late stage, biotite dissolution still continues from the edge and within biotite, which results in a fine comb-like texture for weathered biotite; vermiculite occurs as a domain within chemically and structurally altered biotite as well, as at the edge even during weathering of Mg-poor biotite.

To elucidate development of minerals trapped in a lichen, we examined the lichen Trapelia involuta growing directly on secondary uranyl minerals and U-enriched Fe oxide and hydroxide minerals. Sericite and other minerals in the underlying rock are trapped in the lichen T. involuta during its biological growth and chemically changed by lichen activities. The presence of chemically changed sericite accompanied by an Fe-bearing mineral in the lichen suggests that dissolution of sericite is promoted mainly by polysaccharides excreted by the lichen. Oxalic acid or lichen acids absent in the medulla may not play an important role in the dissolution. Our results suggest that lichens on metal-rich rock surface affect the mobility of uranium and other toxic metals through dissolution followed by trap of minerals from the underlying rock.

To re-investigate “fundamental particles” and “interparticle diffraction” proposed by Nadeau et al. (1984a, 1984b, 1984c), we made physical mixtures of smectite and rectorite and examined them by X-ray diffraction analysis and transmission electron microscopy. Smectite and rectorite consisting of at least 10 and 23 silicate layers in their parent rocks, respectively, reduce their thickness by dispersion in water and a fractionation procedure that releases thin silicate layers from the original crystals. The a-b plane and the particle thickness are appropriate. However, such mixtures do not reflect the actual characteristics of natural smectite and illite. A rectorite particle of about 30 nm thickness consists of sub-particles of two or more rectorite units, or four or more 2:1 layers that are nearly perfectly coherent with one another, and the sub-particles are turbostratically stacked. Our results suggest that natural, interstratified illite/smectite are not accumulations of fundamental particles but represent their structural sequences when formed.