Search Results (66)

The new mineral achyrophanite (K,Na)₃(Fe³⁺,Ti,Al,Mg)₅O₂(AsO₄)₅ was found in high-temperature sublimates of the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with aphthitalite-group sulfates, hematite, alluaudite-group arsenates (badalovite, calciojohillerite, johillerite, nickenichite, hatertite, and khrenovite), ozerovaite, pansnerite, arsenatrotitanite, yurmarinite, svabite, tilasite, katiarsite, yurgensonite, As-bearing sanidine, anhydrite, rutile, cassiterite, and pseudobrookite. Achyrophanite occurs as long-prismatic to acicular or, rarer, tabular crystals up to 0.02 × 0.2 × 1.5 mm, which form parallel, radiating, bush-like, or chaotic aggregates up to 3 mm across. It is transparent, straw-yellow to golden yellow, with strong vitreous luster. The mineral is brittle, with (001) perfect cleavage. D_calc is 3.814 g cm^–3. Achyrophanite is optically biaxial (+), α = 1.823(7), β = 1.840(7), γ = 1.895(7) (589 nm), 2V (meas.) = 60(10)°. Chemical composition (wt.%, electron microprobe) is: Na₂O 3.68, K₂O 9.32, CaO 0.38, MgO 1.37, MnO 0.08, CuO 0.82, ZnO 0.48, Al₂O₃ 2.09, Fe₂O₃ 20.42, SiO₂ 0.12, TiO₂ 7.35, P₂O₅ 0.14, V₂O₅ 0.33, As₂O₅ 51.88, SO₃ 1.04, and total 99.40. The empirical formula calculated based on 22 O apfu is Na_1.29K_2.15Ca_0.07Mg_0.34Mn_0.01Cu_0.11Zn_0.06Al_0.44Fe³⁺_2.77Ti_1.00Si_0.02P_0.02S_0.14V_0.04As_4.90O₂₂. Achyrophanite is orthorhombic, space group P222₁, a = 6.5824(2), b = 13.2488(4), c = 10.7613(3) Å, V = 938.48(5) ų and Z = 2. The strongest reflections of the PXRD pattern [d,Å(I)(hkl)] are 5.615(59)(101), 4.174(42)(022), 3.669(31)(130), 3.148(33)(103), 2.852(43)(141), 2.814(100)(042, 202), 2.689(29)(004), and 2.237(28)(152). The crystal structure of achyrophanite (solved from single-crystal XRD data, R = 4.47%) is unique. It is based on the octahedral-tetrahedral M-T-O pseudo-framework (M = Fe³⁺ with admixed Ti, Al, Mg, Na; T = As⁵⁺). Large-cation A sites (A = K, Na) are located in the channels of the pseudo-framework. The achyrophanite structure can be described as stuffed, with the defect heteropolyhedral pseudo-framework derivative of the orthorhombic Fe³⁺AsO₄ archetype. The mineral is named from the Greek άχυρον, straw, and φαίνομαι, to appear, in allusion to its typical straw-yellow color and long prismatic habit of crystals. Full article

Five requeijão samples, classified as Brazilian cream cheeses, were developed: one control (without guabiroba pulp (Campomanesia xanthocarpa O. Berg) and four with 5, 10, 15, and 20% (m/m) guabiroba pulp. They were evaluated for pH, water activity (aw), color, texture, multi-mineral composition, carotenoid content, and microstructure. The addition of guabiroba pulp reduced pH and maintained Aw. The samples with 5%, 10%, 15%, and 20% guabiroba pulp presented a yellow–reddish coloration. The formulation with 5% had the lowest values of firmness, resilience, texture, and spreadability. From 10% onwards, an increase in cohesiveness and a reduction in creaminess were observed. The sample with 15% presented better spreadability, while the 20% sample had adhesiveness similar to the control. No traces of Al, As, Cd, Co, Cr, Cu, Fe, Mn, Pb, or Se were detected. The detected elements, in descending order, were Na, Ca, P, S, K, Mg, Sr, and Zn. β-carotene was predominant, with guabiroba pulp enhancing α-carotene, β-carotene, β-cryptoxanthin, and λ-carotene levels, especially at 20% pulp. Microstructure analysis by scanning electron microscopy (SEM) showed no significant differences. These findings highlight the potential of guabiroba pulp as a functional ingredient in requeijão, enhancing its carotenoid profile while maintaining desirable textural and physicochemical properties. Full article

A new molecular theory of slag suggests that complex oxides in the phase diagram are also present in liquid slag. In contrast to the ion‒molecule coexistence theory, basic oxides (CaO, MgO, MnO, FeO, etc.) in slag are considered to agglomerate in the liquid state due to their strong mutual attraction, although they are ionized (M²⁺ and O²⁻). The predicted slag structure agrees with the experimental results, and when the model is applied to the CaO-SiO₂, CaO-Al₂O₃, and CaO-SiO₂-Al₂O₃ slag systems, the calculated molar fractions of CaO, SiO₂, and Al₂O₃ ($N_{\mathrm{CaO}},N_{{\mathrm{SiO}}_2},N_{{\mathrm{Al}}_2{\mathrm{O}}_3}$) are close to the measured activities (${\alpha }_{\mathrm{CaO}},a_{{\mathrm{SiO}}_2} \mathrm{and} a_{{\mathrm{Al}}_2{\mathrm{O}}_3}$) reported by different researchers. In the CaO-Al₂O₃ slag system, the results based on the new molecular theory are closer to the experimental values than the results of other theoretical calculations. In the practical application of the new molecular theory, the maximum concentration of each complex molecule is consistent with the position of the melting point of the same solid‒liquid components in the phase diagram, indicating that complex molecules have a strong influence on the melting point of slag. In addition, it is believed that the formation and decomposition of different complex molecules are responsible for changes in component activity in the CaO-SiO₂ and CaO-Al₂O₃ slag systems, and it is further deduced that 3CaO-SiO₂ is formed in two steps. Full article

This study evaluated the effects of three irrigation treatments (control (rain-fed plants), deficit irrigation (DI: 50% of maximum field capacity), full irrigation (FI: 100% of maximum field capacity)), and two crop-management treatments (with or without crop rotation with bean, CR, and NCR, respectively) on the plant growth and chemical composition of C. spinosum. The results indicated that deficit irrigation combined with crop rotation increased the weight of leaves per plant, followed by rain-fed plants for the same crop-management treatment. Additionally, these two factors significantly influenced the nutritional profile, free sugars, and organic acid content in a variable manner. Moreover, the control treatment and deficit irrigation increased the content of K, Na, and Mg, which are highly mobile nutrients, whereas the levels of moderately mobile nutrients such as Fe, Mn, Cu, and Zn decreased. Deficit irrigation without crop rotation significantly increased the content of total tocopherols, followed by deficit irrigation with crop rotation and full irrigation without crop rotation. The main fatty acids were α-linolenic acid (C18:3n3), followed by palmitic acid (C16:0) and linoleic acid (C18:2n6), while the control and deficit irrigation treatments combined with crop rotation increased PUFA and decreased SFA content. Furthermore, deficit irrigation and crop rotation induced the accumulation of phenolic compounds, flavonoids, and phenolic acids, especially the content of the major compounds (e.g., chicoric acid, quercetin-O-hexurunoside, and luteolin-O-hexurunoside). The leaf extracts exhibited varied antioxidant activity (assessed by TBARS and OxHLIA assays), and antimicrobial activity. On the other hand, no antifungal, antiproliferative (except for AGS cell line), hepatotoxic, or anti-inflammatory effects were recorded. In conclusion, the combination of deficit irrigation and crop rotation with bean positively affected the quality traits and the fresh weight of leaves, thus suggesting that such eco-friendly practices could have beneficial effects in the cultivation of C. spinosum plants within the context of climate-change mitigation strategies. Full article

Developing more effective gold–support synergy is essential for enhancing the catalytic performance of supported gold nanoparticles (AuNPs) in the gas-phase oxidation of ethanol to acetaldehyde (AC) at lower temperatures. This study demonstrates a significantly improved Au–support synergy achieved by copper doping in LaMO₃ (M = Mn, Fe, Co, Ni) perovskites. Among the various Au/LaMCuO₃ catalysts, Au/LaMnCuO₃ exhibited exceptional catalytic activity, achieving an AC yield of up to 91% and the highest space-time yield of 764 g_AC g_Au⁻¹ h⁻¹ at 225 °C. Notably, this catalyst showed excellent hydrothermal stability, maintaining performance for at least 100 h without significant deactivation when fed with 50% aqueous ethanol. Comprehensive characterization reveals that Cu doping facilitates the formation of surface oxygen vacancies on the Au/LaMCuO₃ catalysts and enhances Au–support interactions. The LaMnCuO₃ perovskite stabilizes the crucial Cu⁺ species, resulting in a stable Au-Mn-Cu synergy within the Au/LaMnCuO₃ catalyst, which facilitates the activation of O₂ and ethanol at lower temperatures. The optimization of the reaction conditions further improves AC productivity. Kinetic studies indicate that the cleavages of both the O-H bond and the α-C-H bond of ethanol are the rate-controlling steps. Full article

Artificial heterostructures are typically created by layering distinct materials, thereby giving rise to unique characteristics different from their individual components. Herein, two-dimensional α-MnSe nanosheets with a non-layered structure were fabricated on Ga_0.6Fe_1.4O₃ (GFO) films. The superior crystalline properties of MnSe/GFO heterostructures were confirmed through structural and morphological analyses. The remanent polarization is around 1.5 μC/cm² and the leakage current density can reach 2 × 10⁻³ A/cm² under 4 V. In addition, the piezo-response force microscopy amplitude and phase images further supported the ferroelectric property. The significant improvement of coercive field and saturated magnetization, along with the antiparallel signals of Mn and Fe ions observed through synchrotron X-ray analyses, suggest the presence of magnetic interaction within the MnSe/GFO heterostructure. Finally, the excellent photodetector with a photo detectivity of 6.3 × 10⁸ Jones and a photoresponsivity of 2.8 × 10⁻³ A·W⁻¹ was obtained under 532 nm in the MnSe/GFO heterostructure. The characteristics of this heterostructure, which include multiferroic, magnetic exchange bias effect, and photodetection capabilities, are highly beneficial for multifunctional devices. Full article

Polypyrrole (PPy)-based nanocomposite materials are of great interest to the scientific community owing to their usefulness in designing state-of-the-art industrial applications, such as fuel cells, catalysts and sensors, energy devices, and especially batteries. However, the commercialization of these materials has not yet reached a satisfactory level of implementation. More research is required for the design and synthesis of PPy-based composite materials for numerous types of battery applications. Due to the rising demand for environmentally friendly, cost-effective, and sustainable energy, battery applications are a significant solution to the energy crisis, utilizing suitable materials like PPy-based composites. Among the conducting polymers, PPy is considered an important class of materials owing to their ease of synthesis, low cost, environmentally friendly nature, and so on. In this context, PPy-based nanocomposites may be very promising due to their nanostructural properties and distinct morphological topography, which are vital concerns for their applications for battery applications. Such features of PPy-based nanocomposites make them particularly promising for next-generation electrode materials. However, the design and fabrication of appropriate PPy-based nanocomposites for battery applications is still a challenging area of research. This review paper describes the current progress on the synthesizing of PPy-based composites for battery applications along with their morphological topography. We discussed here the recent progress on the synthesis of different PPy-based composites, including PPy/S, PPy/MnOx, MWCNT/PPy, V₂O₅/PPy, Cl-doped PPy/rGO, and Fe/α-MnO₂@PPy composites, by a polymerization approach for numerous battery applications. The insights presented in this review aim to provide a comprehensive reference for the future development of PPy-based composites in battery technology. Full article

The crystal structure, thermal behavior, and vibrational spectra of the anthropogenic analogue of boussingaultite, (NH₄)₂Mg(SO₄)₂·6H₂O, and its dehydrated counterpart efremovite, (NH₄)₂Mg₂(SO₄)₃, were studied in detail. The sample from the Chelyabinsk burning coal dumps has the composition of (NH₄)_1.92(Mg_1.02Mn_0.01Fe_0.01)_∑1.04(SO₄)₂·6H₂O and crystallizes in the space group P2₁/a, with a = 9.3183(4), b = 12.6070(4), c = 6.2054(3) Å, β = 107.115(5)°, V = 696.70(5) ų (at 20 °C), Z = 2. The thermal evolution steps are as follows: boussingaultite (NH₄)₂Mg(SO₄)₂·6H₂O (25–90 °C) → X-ray amorphous phase (100–150 °C) → efremovite (NH₄)₂Mg₂(SO₄)₃ (160–340 °C) → MgSO₄ Cmcm + Pbnm (340–580 °C) → MgSO₄ Pbnm (580–700 °C). Thermal expansion is anisotropic, with the coefficients (×10⁶ °C⁻¹) α₁₁ = 52(2), α₂₂ = 68(2), α₃₃ = –89(3), and α_v = 31(3) at T = –123 °C; and α₁₁ = 53(2), α₂₂ = 67(2), α₃₃ = 15(1), and α_v = 136(3) at T = 60 °C. The maximal thermal expansion is along the b-axis and is due to straightening of corrugated pseudolayers (within the ab plane) of Mg(H₂O)₆ octahedra and SO₄ tetrahedra with NH₄ groups in the interlayer space. Vibrational spectroscopy outlines the general trend of dehydration and deammonization as the difference in the temperature intervals of these transformation steps allows separation of O–H and N–H vibrations in the process of dehydration by infrared and Raman spectroscopy. The intermediate partially dehydrated modification of boussingaultite was detected by in situ Raman spectroscopy at 110 °C that may correspond to ammonium leonite. Full article

MnZn ferrite powders were prepared based on the novel nano in situ composite method and through chemical sol-spray drying–calcination technology. The precursor powders were calcined at 1060 °C at different calcination times (1–9 h) to research the influences of the calcination time on MnZn ferrite powders. The research results revealed that all samples had similar morphologies composed of fine particles. The pure MnZn ferrite spinel phase can only be obtained when the calcination time does not exceed 3 h. Otherwise, some α-Fe₂O₃ or γ-Fe₂O₃ impurities will appear. The particle size descended with an increasing calcination time and then ascended. After 3 h of preservation, the smallest particle size was obtained, and it exhibited a unimodal distribution. The saturation magnetization (M_s) increased at first and decreased later with an increasing calcination time, and the optimal value (53.4 emu/g) was reached after holding for 3 h. In view of this work, the optimal calcination time is 3 h. Full article

A Ga isotope is a new proxy for different geochemical processes such as a weathering process, solution process, etc. Si (Si(OH)₄) is ubiquitous in natural water bodies. However, studies on the Ga isotope effect about a Ga³⁺ aqueous solution reacting with Si (Si(OH)₄) are lacking. In this study, the Ga isotope effect of this process will be studied using a theoretical calculation method based on first principles. The results show that the heavy Ga (⁷¹Ga) isotope enrichment ability of different Ga-Si complex solutions is different. The 1000lnβ (‰) sequence of different Ga-Si complex solutions is (OH)₃GaOSi(OH)₃.(H₂O)₃₀ ≈ (OH)₃(H₂O)₂GaOSi(OH)₃.(H₂O)₃₀ > (OH)₂(H₂O)₃GaOSi(OH)₃.(H₂O)₃₀ > (H₂O)₅GaOSi(OH)₃.(H₂O)₃₀ > (OH)(H₂O)₄GaOSi(OH)₃.(H₂O)₃₀. The results show that there are two different reaction mechanisms when a Ga³⁺ aqueous solution reacts with Si-bearing (Si(OH)₄) water; that is, six-coordination Ga-Si complexes and four-coordination Ga-Si complexes are formed at low pH (acidic) and high pH (alkaline), respectively. Compared with a Ga-Si complex aqueous solution under acidic conditions, Ga-Si aqueous solutions under alkaline conditions preferentially enriched the heavy Ga isotope (⁷¹Ga). The Ga isotope fractionation factors (α) between Ga-Si complex solutions and Ga³⁺-bearing aqueous solutions are all negative, which indicates that light Ga (⁶⁹Ga) isotopes preferentially enter the structure of Ga-Si complexes during the formation of Ga-Si complex solutions. At 50 °C, the Ga isotope fractionation factors (1000lnα) of five systems ((H₂O)₅GaOSi(OH)₃.(H₂O)₃₀ vs. [Ga(H₂O)₆]³⁺_(aq), (OH)(H₂O)₄GaOSi(OH)₃.(H₂O)₃₀ vs. [Ga(H₂O)₆]³⁺_(aq), (OH)₃GaOSi(OH)₃.(H₂O)₃₀ vs. [Ga(OH)₃](aq), (OH)₃(H₂O)₂GaOSi(OH)₃.(H₂O)₃₀ vs. [Ga(OH)₃]_(aq), and (OH)₂(H₂O)₃GaOSi(OH)₃.(H₂O)₃₀ vs. [Ga(OH)₃]_(aq)) involved in this study are −0.12, −0.22, −0.07, −0.09, and −0.16 (‰), respectively. Excitedly, Si can affect the enrichment ability of the heavy Ga isotope (⁷¹Ga) in Ga-bearing complex aqueous solutions. This means that when Si is present in aqueous solutions, the enrichment capacity of the heavy Ga isotope (⁷¹Ga) of aqueous solutions will be effectively reduced. Ga in sediments is mainly derived from soluble Ga in the form of adsorbed (Fe, Mn) oxides/hydroxides, and the Ga isotope composition in sediments is heavier than that in basalt. The formation process of Ga-Si complex aqueous solutions influences the Ga isotope fractionation effect and also contributes to the composition of Ga isotopes in sediments. These key Ga isotope fractionation parameters obtained in this study will provide theoretical support for better explaining the reaction mechanism of Ga³⁺ complexes and Si-bearing (Si(OH)₄) water bodies in solution processes and Ga isotope geochemical cycles. Full article

This study explores the deposition of an Fe-MnCrSi/TiC coating on 45 steel surfaces using high-velocity arc spraying technology, examining the microstructure and hot corrosion behavior of the resultant layer. The microstructure of the FeMnCrSi/TiC coating primarily consists of an α-Fe (BCC) solid solution, composed of Fe, Mn, Cr, Si, C, and other elements, with a minor presence of β-Fe (FCC) solid-solution phase and unmelted TiC particles. Following 100 h of cyclic 900 °C hot corrosion, Mn on the coating surface preferentially oxidizes, forming a manganese-rich oxide layer. This process reduces the oxygen partial pressure (O₂) within the coating, prompting the formation of a dense Cr₂O₃ layer on the inner side of the oxide layer. Concurrently, the rapid diffusion of Mn and Cr elements triggers the generation of Mn- and Cr-deficient regions at the metal/oxide layer interface, inducing the transformation of the coated metal primary matrix from an FCC + BCC dual phase to an α-Fe (BCC) single phase. After the reaction, the hot corrosion weight gain of the coating reached 12.43 mg/cm², approximately one-fourteenth of the weight gain of the 45 steel substrates. This weight gain adheres to the parabolic law, suggesting that the FeMnCrSi/TiC coating exhibits excellent corrosion resistance under the given conditions. Full article

Corrosion has always been an important factor affecting the life of steel, which causes huge economic losses every year. How to improve the corrosion resistance of steel has always been a research focus. Adding rare earth elements into steel is an important method to improve the corrosion resistance of steel. In this paper, the effects of rare earth elements on steel are summarized, including the purification of molten steel, modification and modification of inclusions, improvement of grain boundaries by solid solution strengthening, the influence of phase transformation and the refinement of microstructure, and reduction in C and N desolubilization. On this basis, the progress of research on the corrosion resistance mechanisms of rare earth steel is summarized, focusing on rare earth-modified inclusions. This includes the changes in composition and sizes of inclusions by rare earth addition, promoting the transformation of MnS and Al₂O₃ in rare earth inclusions with regular shapes, smaller sizes and better performance, or composite rare earth inclusions. The corrosion pits that form in the early stages of corrosion are shallow in depth, fewer in number and light in corrosion degree. The effects of rare earth materials on the rust layer include: rare earth promotes the formation of a more stable corrosion product α-FeOOH, and rare earth promotes the formation of a dense rust layer, which covers the surface of the matrix and hinders the transmission of corrosion ions. The protective effect of the rare earth atomic layer on the substrate and the corrosion inhibition effect of rare earth ions are formed by the segregation of rare earth at the interface. In the end, the existing problems in the research into rare earth steel and future research directions are briefly put forward, including improving the addition process of rare earth steel, theoretical guidance on enhancing the corrosion resistance mechanism of rare earth steel, and extending the research from La, Ce, and Y steel to more rare earth steels. Full article

In this study, FeMnCrNiBNbAl wear-resistant coatings were prepared via high-velocity arc spraying technology on the surface of Q235 steel. The effects of spraying distance, voltage, and current on the coating performance were studied via the orthogonal experimental method with microhardness and porosity as evaluation indicators, and the process parameters were optimized. The order of primary and secondary factors affecting coating performance were: spraying distance, voltage, and current. The optimized process parameters are: spraying distance 200 mm, voltage 36 V, and current 240 A. The coating prepared using the optimized process parameters has an average microhardness of 756 HV0.1 and an average porosity of 1.03%. The coating mainly consists of α-Fe, Fe-Cr, Ni-Cr-Fe, Al₂O₃, and (Fe, Ni) solid solution. The coating friction coefficient was 0.5 while that of the substrate was 0.7. The depth and width of the wear marks of the coating were 6.32 µm and 555.41 µm, respectively, which are 66% and 32% lower in comparison with Q235 steel under the same test conditions. The wear forms of this coating were mainly fatigue peeling and adhesive wear. Full article

The practical application of rechargeable aqueous zinc-ion batteries (ZIBs) has been severely hindered by detrimental dendrite growth, uncontrollable hydrogen evolution, and unfavorable side reactions occurring at the Zn metal anode. Here, we applied a Prussian blue analogue (PBA) material K₂Zn₃(Fe(CN)₆)₂ as an artificial solid electrolyte interphase (SEI), by which the plentiful -C≡N- ligands at the surface and the large channels in the open framework structure can operate as a highly zincophilic moderator and ion sieve, inducing fast and uniform nucleation and deposition of Zn. Additionally, the dense interface effectively prevents water molecules from approaching the Zn surface, thereby inhibiting the hydrogen-evolution-resultant side reactions and corrosion. The highly reversible Zn plating/stripping is evidenced by an elevated Coulombic efficiency of 99.87% over 600 cycles in a Zn/Cu cell and a prolonged lifetime of 860 h at 5 mA cm⁻², 2 mAh cm⁻² in a Zn/Zn symmetric cell. Furthermore, the PBA-coated Zn anode ensures the excellent rate and cycling performance of an α-MnO₂/Zn full cell. This work provides a simple and effective solution for the improvement of the Zn anode, advancing the commercialization of aqueous ZIBs. Full article

A Type I reaction center (RC) (Fe-S type, ferredoxin reducing) is found in several phyla containing anoxygenic phototrophic bacteria. These include the heliobacteria (HB), the green sulfur bacteria (GSB), and the chloracidobacteria (CB), for which high-resolution homodimeric RC-photosystem (PS) structures have recently appeared. The 2.2-Å X-ray structure of the RC-PS of Heliomicrobium modesticaldum revealed that the core PshA apoprotein (PshA-1 and PshA-2 homodimeric pair) exhibits a structurally conserved PSI arrangement comprising five C-terminal transmembrane α-helices (TMHs) forming the RC domain and six N-terminal TMHs coordinating the light-harvesting (LH) pigments. The Hmi. modesticaldum structure lacked quinone molecules, indicating that electrons were transferred directly from the A₀ (8¹-OH-chlorophyll (Chl) a) acceptor to the F_X [4Fe-4S] component, serving as the terminal RC acceptor. A pair of additional TMHs designated as Psh X were also found that function as a low-energy antenna. The 2.5-Å resolution cryo-electron microscopy (cryo-EM) structure for the RC-PS of the green sulfur bacterium Chlorobaculum tepidum included a pair of Fenna–Matthews–Olson protein (FMO) antennae, which transfer excitations from the chlorosomes to the RC-PS (PscA-1 and PscA-2) core. A pair of cytochromes c_Z (PscC) molecules was also revealed, acting as electron donors to the RC bacteriochlorophyll (BChl) a’ special pair, as well as PscB, housing the [4Fe-4S] cluster F_A and F_B, and the associated PscD protein. While the FMO components were missing from the 2.6-Å cryo-EM structure of the Zn- (BChl) a’ special pair containing RC-PS of Chloracidobacterium thermophilum, a unique architecture was revealed that besides the (PscA)₂ core, consisted of seven additional subunits including PscZ in place of PscD, the PscX and PscY cytochrome c serial electron donors and four low mol. wt. subunits of unknown function. Overall, these diverse structures have revealed that (i) the HB RC-PS is the simplest light–energy transducing complex yet isolated and represents the closest known homolog to a common homodimeric RC-PS ancestor; (ii) the symmetrically localized Ca²⁺-binding sites found in each of the Type I homodimeric RC-PS structures likely gave rise to the analogously positioned Mn₄CaO₅ cluster of the PSII RC and the Tyr_Z RC donor site; (iii) a close relationship between the GSB RC-PS and the PSII Chl proteins (CP)43 and CP47 was demonstrated by their strongly conserved LH-(B)Chl localizations; (iv) LH-BChls of the GSB-RC-PS are also localized in the conserved RC-associated positions of the PSII Chl_Z-D1 and Chl_Z-D2 sites; (v) glycosylated carotenoids of the GSB RC-PS are located in the homologous carotenoid-containing positions of PSII, reflecting an O₂-tolerance mechanism capable of sustaining early stages in the evolution of oxygenic photosynthesis. In addition to the close relationships found between the homodimeric RC-PS and PSII, duplication of the gene encoding the ancestral Type I RC apoprotein, followed by genetic divergence, may well account for the appearance of the heterodimeric Type I and Type II RCs of the extant oxygenic phototrophs. Accordingly, the long-held view that PSII arose from the anoxygenic Type II RC is now found to be contrary to the new evidence provided by Type I RC-PS homodimer structures, indicating that the evolutionary origins of anoxygenic Type II RCs, along with their distinct antenna rings are likely to have been preceded by the events that gave rise to their oxygenic counterparts. Full article