The present work uncovers the geochemical control on the nature (tunnel size) of the tectomanganates formed from layered precursors, and thus provides insights into the formation of Mn oxides in natural environments. Large tunnel sizes are favored under circum-neutral conditions, whereas low pH conditions favor the formation of tectomanganates with smaller tunnel sizes. Both the increased proportions of Mn(III) in vernadite/birnessite layers resulting from low pH conditions and the subsequent enhancement of Mn(III) disproportionation during subsequent transformation contribute to the formation of tectomanganates with smaller tunnel sizes. The fate of foreign elements during the phyllomanganate-to-tectomanganate mineral transformation is another important aspect of this mineral transformation, together with the impact of these elements on the transformation. Layered and tunnel Mn oxides have indeed a pivotal influence on the geochemical cycling of transition metals, including Co, that possess a strong affinity for these mineral species. The present experimental work shows that the formation of todorokite (3 × 3 tunnel size), hollandite (2 × 2), or nsutite (intergrown 1 × 1 and 1 × 2 fragments) is essentially unaffected by limited Co-enrichment (≤5 at.%) of the initial phyllomanganate structure. Higher Co contents reduce the content of Jahn-Teller distorted Mn(III) octahedra in layered precursor and hamper the phyllomanaganate-to-tectomanganate transformation. Finally, Co is retained in the structure of todorokite and hollandite during their formation under circum-neutral conditions whereas part (∼20%) of the Co present in layered precursors is expelled out of the framework and/or sorbed to nsutite formed under acidic conditions. This effect is induced by the reduced stability of Co(III) octahedra when the relative proportion of corner-sharing linkages increases. In turn, this effect influences Co structural incorporation in different Mn oxides and its potential release to solution.

目前的工作揭示了对由层状前驱物形成的锰锰酸盐的性质（隧道尺寸）的地球化学控制，从而提供了对自然环境中锰氧化物形成的见解。在环境中性条件下，较大的通道尺寸是有利的，而在较低的pH条件下，有利于形成较小通道尺寸的四锰锰酸盐。低pH条件导致的白云母/水钠锰矿层中Mn（III）比例的增加以及后续转化过程中Mn（III）歧化度的增加都有助于形成具有较小隧道尺寸的钛锰酸盐。叶状锰酸盐向尾矿锰酸盐矿物转化过程中外来元素的命运是这种矿物转化的另一个重要方面，以及这些要素对转型的影响。层状和隧道锰氧化物确实对过渡金属（包括Co）的地球化学循环具有关键影响，过渡金属对这些矿物质具有很强的亲和力。目前的实验工作表明，钙钛矿（3×3隧道尺寸），白铁矿（2×2）或nsutite（互生的1×1和1×2碎片）的形成基本上不受有限的富集（≤5％ （％）的初始叶锰酸根结构。较高的Co含量会降低层状前体中Jahn-Teller变形的Mn（III）八面体的含量，并阻碍了叶锰酸根向钛锰酸根的转化。最后，在中性环境条件下形成过程中，Co保留在钙锰矿和堇青石的结构中，而层状前体中存在的一部分Co（约20％）则从骨架中排出和/或吸附到在酸性条件下形成的辉绿岩中。当角共享链接的相对比例增加时，Co（III）八面体的稳定性降低会导致这种效果。反过来，这种效应影响了在不同的Mn氧化物中Co结构的结合及其向溶液中的潜在释放。