The chemistry of bridgmanite (Brg), especially the oxidation state of iron, is important for understanding the physical and chemical properties, as well as putting constraints on the redox state, of the Earth’s lower mantle. To investigate the controls on the chemistry of Brg, the Fe³⁺ content of Brg was investigated experimentally as a function of composition and oxygen fugacity (fo₂) at 25 GPa. The Fe³⁺/∑Fe ratio of Brg increases with Brg Al content and fo₂ and decreases with increasing total Fe content and with temperature. The dependence of the Fe³⁺/∑Fe ratio on fo₂ becomes less steep with increasing Al content. Thermodynamic models were calibrated to describe Brg and ferropericlase (Fp) compositions as well as the inter-site partitioning of trivalent cations in Brg in the Al–Mg–Si–O, Fe–Mg–Si–O and Fe–Al–Mg–Si–O systems. These models are based on equilibria involving Brg components where the equilibrium thermodynamic properties are the main adjustable parameters that are fit to the experimental data. The models reproduce the experimental data over wide ranges of fo₂ with a relatively small number of adjustable terms. Mineral compositions for plausible mantle bulk compositions can be calculated from the models as a function of fo₂ and can be extrapolated to higher pressures using data on the partial molar volumes of the Brg components. The results show that the exchange of Mg and total Fe (i.e., ferric and ferrous) between Brg and Fp is strongly fo₂ dependent, which allows the results of previous studies to be reinterpreted. For a pyrolite bulk composition with an upper mantle bulk oxygen content, the fo₂ at the top of the lower mantle is −0.86 log units below the iron–wüstite buffer (IW) with a Brg Fe³⁺/∑Fe ratio of 0.50 and a bulk rock ratio of 0.28. This requires the formation of 0.7 wt.% Fe–Ni alloy to balance the raised Brg ferric iron content. With increasing pressure, the model predicts a gradual increase in the Fe³⁺/∑Fe ratio in Brg in contrast to several previous studies, which levels off by 50 GPa. Oxygen vacancies in Brg decrease to practically zero by 40 GPa, potentially influencing elasticity, diffusivity and rheology in the top portion of the lower mantle. The models are also used to explore the fo₂ recorded by inclusions in diamonds, which likely crystallized as Brg in the lower mantle, revealing oxygen fugacities which likely preclude the formation of some diamonds directly from carbonates, at least at the top of the lower mantle.

桥锰矿（Brg）的化学性质，尤其是铁的氧化态，对于理解地球下地幔的物理和化学特性以及对氧化还原态施加约束非常重要。为了研究Brg的化学控制，实验研究了25 gPa下Brg的Fe ³⁺含量与组成和氧逸度（f o ₂）的关系。Brg的Fe ³⁺ / ∑Fe比随Brg Al含量和f o _2的增加而增加，随总Fe含量和温度的增加而降低。Fe ³⁺ / ∑Fe比对f o ₂的依赖性随着Al含量的增加，陡度变小。校准热力学模型以描述Brg和阿魏酸酯酶（Fp）的组成，以及在Al–Mg–Si–O，Fe–Mg–Si–O和Fe–Al–Mg–Al中Brg中三价阳离子的位点间分配Si–O系统。这些模型基于涉及Brg组分的平衡，其中平衡热力学性质是适合实验数据的主要可调参数。这些模型以相对较少的可调整项在f o _2的较大范围内再现了实验数据。可以根据模型根据f o ₂来计算可能的地幔散装组成的矿物组成并且可以使用有关Brg组分的部分摩尔体积的数据推断出较高的压力。结果表明，Brg和Fp之间的Mg和总Fe（即三价铁和二价铁）的交换强烈依赖于f o ₂，这使得以前的研究结果得以重新解释。对于具有较高地幔体积氧含量的黄铁矿大体积成分，在较低地幔顶部的f o ₂在铁-锰铁矿缓冲剂（IW）下方为−0.86 log单位，Brg Fe ³⁺ / ∑Fe比为0.50体积比为0.28。这要求形成0.7 wt。％的Fe-Ni合金以平衡所增加的Brg三价铁含量。随着压力的增加，该模型预测铁的含量会逐渐增加。与以前的几项研究相反，Brg中的³⁺ / ∑Fe比值稳定在50 GPa。Brg中的氧空位通过40 GPa降低到几乎为零，可能影响下地幔顶部的弹性，扩散性和流变性。这些模型还用于探查钻石夹杂物所记录的f o ₂，其可能在下地幔中结晶为Brg，揭示出氧逸散性，这可能阻止了某些碳酸盐直接从碳酸盐中至少在下部的顶部形成某些钻石。地幔。