Structure, Formation and Transformation of Minerals

Minerals are linked in many important ways with the global ecosystem. Minerals are the main sources of elements needed for the development of civilization and elements that are pollutants or essential plant nutrients. These elements are released from minerals through chemical weathering and anthropogenic activities, such as mining and energy production. Minerals also play key roles in the biogeochemical cycling of the elements, sequestering elements and releasing them as the primary minerals in crustal rocks undergo various structural and compositional transformations in response to physical, chemical, and biological processes that produce secondary minerals and soils. The mineral-water interfaces are the locations of most chemical reactions (e.g., adsorption/desorption, oxidation/reduction and precipitation/dissolution) that control the composition of the natural environment, including the composition of natural waters. To improve the fundamental understanding of these processes, our group uses modern molecular-scale analytical and theoretical methods to acquire molecular-scale information about minerals, including their bulk structures and properties, formation and transformation. The minerals of our primary interests are Mn and Fe oxides that are highly reactive and actively participate in complex redox and mineral-water interfacial reactions. These reactions strongly affect biogeochemical cycles and pollutant dynamics in Earth's critical zone (CZO).

Our current focus is on birnessite minerals that are the most common and abundant type of layered Mn(IV) oxides in nature, imposing significant impact on many critical biogeochemical processes owing to their extraordinary sorption and oxidation properties. We try to address what and how geochemial processes affect birnessite chemical composition (e.g., vacancy and Mn(III) concentrations) and particle sizes, which are birnessite reactivity-determining factors, and its transformation to tunneled Mn(IV) oxides. Some of our recent work are introduced below.

1) Metal Adsorption Controls Stability of Layered Manganese Oxides (Yang et al., Environ. Sci. Technol., 2019)

Hexagonal birnessite, a typical layered Mn oxide (LMO), can adsorb and oxidize Mn(II) and thereby transform to Mn(III)-rich hexagonal birnessite, triclinic birnessite, or tunneled Mn oxides (TMOs), remarkably changing the environmental behavior of Mn oxides. We have determined the effects of coexisting cations on the transformation by incubating Mn(II)-bearing δ-MnO2 at pH 8 under anoxic conditions for 25 d (dissolved Mn < 11 μM). In the Li+, Na+, and K+ chloride solutions, the Mn(II)-bearing δ-MnO2 first transforms to Mn(III)-rich δ-MnO2 or triclinic birnessite (T-bir) due to the Mn(II)-Mn(IV) comproportionation, most of which eventually transform to a 4 Χ 4 TMO. In contrast, Mn(III)-rich δ-MnO2 and T-bir form and persist in the Mg2+ and Ca2+ chloride solutions. However, in the presence of surface adsorbed Cu(II), Mn(II)-bearing δ-MnO2 turns into Mn(III)-rich δ-MnO2 without forming T-bir or TMOs. The stabilizing power of the cations on the δ-MnO2 structure positively correlates with their binding strength to δ-MnO2 (Li+, Na+, and K+ < Mg2+ and Ca2+ < Cu(II)). Since metal adsorption decreases the surface energy of minerals, our finding suggests that the surface energy largely controls the thermodynamic stability of LMOs. Our study indicates that the adsorption of divalent metal cations, particularly transition metals, can be an important cause of the high abundance of LMOs, rather than the more stable TMO phases, in the environment.

Transformation pathways of Mn(II)-bearing δ-MnO2 to other phases in different systems.

2) Trivalent manganese on vacancies triggers rapid transformation of layered to tunneled manganese oxides (TMOs): Implications for occurrence of TMOs in low-temperature environment (Yang et al., Geochim. Cosmochim Acta., 2018)

Tunneled Mn oxides (TMOs) are common minerals in natural environment, particularly in ferromanganese nodules of oceanic and lake sediments. Their structures host a considerable amount of transition and rare earth metals, thus mediating metal cycling and bearing potential economic interest for exploiting these metals. TMOs form through topotactic transformation of layered Mn oxides (LMOs), such as vernadite, in natural environment. Trivalent Mn (Mn(III)) in the LMO structure is a critical player in the transformation, and the transformation is believed to be extremely slow at room temperature. However, the specific role of Mn(III) and its impacts on the transformation kinetics remain unknown. In the present study, we show that the formation of Mn(III) on vacancies of an LMO is the initial transformation step leading to TMOs, and that the transformation can be rapid at room temperature and circumneutral pH. Specifically, after pre-adsorbed with Mn(II) on vacancies at pH 4, δ-MnO2, a hexagonal birnessite analogous to vernadite, starts to transform to a 4 Χ 4 TMO at  1 h upon incubation at pH 7 and 21 oC under anoxic conditions. The rapid transformation is triggered by the comproportionation reaction between the vacancy-adsorbed Mn(II) and Mn(IV) in δ-MnO2 that produces Mn(III) on the vacancies. Such intermediate Mn(III)-rich product acts as a precursor for subsequent rapid structural rearrangement to form tunnels. An incubation at lower or higher pH retards the transformation due to an insufficient amount of Mn(III) (pH 6) or the formation of triclinic birnessite (pH 8) as an intermediate product. The presence of O2 favors the formation of triclinic birnessite at pH 8 and thus retards the transformation whereas O2 enhances production of Mn(III)-rich hexagonal birnessite at pH 6 and 7 and promotes the transformation. We propose a novel transformation mechanism of LMOs to TMOs, highlighting the role of vacancy-adsorbed Mn(III) in the transformation. This work changes our understanding of TMO formation kinetics and suggests TMOs can readily form in low-temperature redox-fluctuating environment, such as lake and oceanic sediments where Mn(II) often coexists with LMOs.


Three potential transformation pathways from LMOs to TMOs. (a) An interlayer-condensation mechanism for transformation of Mn(II)-bir to the 4 X 4 TMO involves steps 1-4 (see text). pH controls which phase to form, a TMO phase (Steps 2-4) or triclinic birnessite (T-bir, Step 2). (b) T-bir transforms to the TMO phase either through a folding mechanism (Steps 1-3), or the interlayer-condensation mechanism (Steps 2-4) that requires layered Mn(III) first to migrate onto vacancies (Step 1). For the folding mechanism, the Mn(III) and Mn(IV) rows first rearrange so that every four rows of Mn contain one or two Mn(III) rows located at the edges. The rearrangement is likely through intra-particle electron transfer. Second, the outermost layers fold in the sigmoidal/reverse-sigmoidal way along the weakened Mn(III)-O-Mn(IV) boundaries to form a nucleus of 4X4 tunnels. Finally, other layers follow zigzag folding along the edge of the nucleus to form 4X4 TMO particles. (c) Mn(II)-bir transforms into 1X1 and 1X2 TMOs via the production of vacancy-adsorbed Mn(III) (Step 1), and the Mn(III) disproportionation to form Mn(IV) on vacancies (Step 2) followed by the structural arrangement (Step 3) at low pHs. In this pathway, Mn(III) is a transient species and Mn(II) is a catalyst.

3) Effects of metal cations on coupled birnessite structural transformation and natural organic matter adsorption and oxidation (Wang et al. Geochim. Cosmochim. Acta., 2019)

Birnessite, a layered manganese (Mn) oxide, possesses extraordinary metal adsorption and oxidation activity, and thus imposes impacts on many biogeochemical processes. The reactivity of birnessite strongly relies on its Mn oxidation state composition (the proportions of Mn II, III and IV), particularly by the Mn(III) proportion. Partial reduction of birnessite transforms birnessite to be Mn(II, III)-rich or to MnOOH and Mn3O4, and thus strongly affects birnessite reactivity. As a metal scavenger, naturally occurring birnessite contains abundant transition and alkali and alkaline earth metal cations in its structure; however, the effects of these metal cations on the partial reduction-induced transformation of birnessite remain unknown. We examined the effects of Zn2+, Mg2+, Ca2+ and ionic strength (controlled by NaCl) on transformation of birnessite (δ-MnO2) and adsorption and oxidation of natural organic matter during partial reduction by fulvic acid (FA) at pH 8 and FA/MnO2 mass ratios (R) of 0.1 or 1 over 600 h under anoxic conditions. Results showed that low ionic strength (0 versus 50 mM NaCl) disfavored FA adsorption, fractionation and oxidation, and thus disfavored formation of Mn(III) in the reacted birnessite. Compared to the 50 mM NaCl system, all divalent cations (Mg2+, Ca2+ and Zn2+) favored FA adsorption and fractionation. Both Mg2+ and Ca2+ significantly enhanced FA oxidation at the early stage but barely at the late stage, whereas Zn2+ strongly suppressed FA oxidation during the entire experimental period. Due to adsorption competition, the presence of the divalent cations resulted in low concentration of Mn(II) adsorbed on vacancies of birnessite. Both Ca2+ and Mg2+ favored Mn(III) production in MnO6 layers, while Zn2+ inhibited it. A small portion of birnessite also transformed to feitknechtite and hausmannite, and the transformation seemed faster in the presence of Ca2+ or Mg2+ than in NaCl solution. In the presence of Zn2+ at the high FA/MnO2 ratio (R=1), Zn-substituted hansmannite formed extenstively. The formation of Mn(III) in the reacted birnessite can be ascribed to comproportionation between Mn(IV) and Mn(II) adsorbed on either vacancies or edge sites of birnessite. The low-valence Mn oxide phases likely formed via the comproportionation on the edges. The divalent cations affected Mn(III) concentrations of birnessite and formation of the low-valence Mn oxides by competing with Mn(II) for adsorption on edge/vacancy sites or stabilizing Mn(III) in the layers. This work indicates that divalent metal cations strongly influence reactivity and transformation of birnessite in the coupled Mn and carbon redox cycles, and that birnessite containing divalent cations can be an important adsorbent for natural organic carbon in Mn-rich environments. Overall, this study provides insights into the coupled cycles of Mn, trace metals and organic carbon in alkaline and saline environments.

A proposed mechanism for the formation of Mn(III)-rich birnessite and MnOOH/Mn3O4 phases at circumneutral pHs via comproportionation between Mn(IV) in birnessite and Mn(II). At low Mn(II)/birnessite ratios, Mn(II) adsorbs on vacancy sites and the comproportionation reaction between Mn(IV) and vacancy-adsorbed Mn(II) produces Mn(III)-rich birnessite. The newly-formed vacancy-adsorbed Mn(III) can also enter the vacancy to be part of the layer. With increasing Mn(II)/birnessite ratio, Mn(II) starts to adsorb on edge sites, and comproportionation between Mn(IV) and edge-adsorbed Mn(II) produces Mn(III). The accumulated Mn(III) on the edge sites may redistribute in the layers via dynamic atomic exchange and intra-layer Mn(III)-Mn(IV) electron transfer. At a high Mn(II)/birnessite ratio, the Mn(III) on edges further rearrange to form MnOOH phases, or react with additional Mn(II) to form Mn3O4.