The valence and spin states of Fe in (Fe,Al)-bearing bridgmanite (bdg) affect its physical properties, which is important for the interpretation of geophysical observations. Currently, tens of studies on the compressibility and spin states of Fe-bearing bdg have been reported. A consensus is that Fe-bearing bdg shows spin transition, which affects its elastic parameters. However, there is a conflict between reports on the compressibility and spin states of (Fe,Al)-bearing bdg in experiments using samples pre-synthesized in a multi-anvil apparatus (MA), and samples directly synthesized in a diamond anvil cell (DAC). There are no reports showing evidence of spin transition of Fe in compression experiments using (Fe,Al)-bearing bdg samples pre-synthesized in a MA, while those synthesized at relatively high pressure (at least above 45 GPa) in a DAC all exhibited the spin transition. Here, we performed synchrotron X-ray diffraction measurements on Mg0.85Fe0.09Al0.21Si0.86O3 and Mg0.85Fe0.14Al0.05Si0.96O3 bdg synthesized at relatively high pressure in a laser-heated DAC from amorphous starting material up to 47 and 56 GPa, respectively, at room temperature. The obtained pressure (P)–lattice volume (V) relations show noticeable softening at 22–30 GPa and 35–45 GPa, respectively, which is probably due to the spin transition of Fe. Combining our results and previous reports, we suggest that the lower mantle bdg is capable of containing low-spin Fe3+, which questions the general view. Such a transition changes density and may affect the physical properties of bridgmanite such as thermal conductivity and iron partitioning coefficient, thus having profound implications for mantle dynamics, and the chemical composition of the Earth.

中文翻译：

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含 (Fe,Al) 的布里奇曼石的异常可压缩性：对铁自旋状态的影响

含 (Fe,Al) 的桥锰矿 (bdg) 中 Fe 的价态和自旋态会影响其物理性质，这对于解释地球物理观测很重要。目前，已报道了数十项关于含铁 bdg 的可压缩性和自旋态的研究。一个共识是含铁 bdg 显示自旋转变，这会影响其弹性参数。然而，在使用多砧装置 (MA) 中预先合成的样品的实验中，关于 (Fe,Al) 轴承 bdg 的可压缩性和自旋状态的报告与在金刚石砧座中直接合成的样品之间存在冲突。 DAC）。没有报告显示在使用 MA 中预先合成的含 (Fe,Al) 的 bdg 样品的压缩实验中 Fe 自旋跃迁的证据，而那些在 DAC 中在相对较高的压力（至少高于 45 GPa）下合成的化合物都表现出自旋跃迁。在这里，我们对在激光加热 DAC 中从高达 47 和在室温下分别为 56 GPa。获得的压力（P）-晶格体积（V）关系分别在 22-30 GPa 和 35-45 GPa 处显着软化，这可能是由于 Fe 的自旋转变。结合我们的结果和之前的报告，我们认为下地幔 bdg 能够包含低自旋 Fe3+，这对一般观点提出了质疑。