Mineral Membrane on Earth’s surface and Natural Mineral Photoelectric Effect

Abstract:

Innumerable previous studies concerning the most critical influences of solar energy on the surface of this planet have mainly focused on its influence on Earth’s climate, on photosynthesis and associated biological processes, and on geological and soil processes. However, although widely exposed natural minerals on Earth’s surface receive solar irradiation over very long time periods, the electronic response of these interactions and how this process also shapes the planet on which we live have rarely received attention. In this work, we reveal the inherent “phototrophic-like” behavior of vast expanses of natural rock/soil surfaces from deserts, red soils and karst environments, all of which can drive photon-to-electron conversions. Fe and Mn (oxyhydr)oxide-rich membrane was found in rock varnishes, as were Fe (oxyhydr)oxides on red soil surfaces and minute amounts of Mn oxides on karst rock surfaces. By directly fabricating a photoelectric detection device on the thin section of a rock varnish sample, we have recorded the first in situ photocurrent micromapping of the membrane, which behave as highly sensitive and stable photoelectric systems. Additional measurements of red soil and powder separated from the outermost surface of karst rocks yielded photocurrents that are also sensitive to irradiation. The prominent solar-responsive capability of the phototrophic-like rocks/soils is ascribed to the semiconducting Fe and Mn (oxyhydr)oxide mineral membrane. The native semiconducting Fe/Mn-rich membrane may play a role similar in part to photosynthetic systems and thus provide a distinctive driving force for redox (bio)geochemistry on Earth’s surfaces.

We also discovered for the first time the solar-energy conversion system of the “mineral membrane”, which exerts potential oxygen-production and carbon-sequestration function on Earth surface. Our finding shed light on the photo-electric effect and non-classic photo-synthesis of the natural semiconducting minerals. This research was based on the semiconducting property and photo-electron energy of the typical minerals in “mineral membrane”, and further discussed the photo-electric effect, oxygen-production and carbon-sequestration function of the ferromanganese oxides, as well as necessarily corresponding geological records. Our study proposed the sensitive and stable photon-to-electron conversion performed by birnessite, goethite and hematite, which are semiconducting minerals commonly present in the natural mineral membrane. In addition, the non-classic mineral photosynthesis function was put forward: the solar-energy conversion system developed by inorganic minerals resembled the biological photosynthesis process, as to oxygen evolving and carbon fixing; also, mineral membrane could take part in the photo-catalytic water-oxidation reaction, and the transformation of atmospheric CO₂ into marine carbonate. Last but not least, minerals could as well promote the biologic photosynthesis system: the core complex in the photosynthesis system Mn₄CaO₅ evolved during water-oxidation process to the structural analog as birnessite. Therefore, it is fair to postulate that birnessite might possibly play a role in the initiation of the photosynthesis system of cyanobacteria.

In solar-terrestrial systems, solar energy input has long been recognized to have a profound impact on Earth. The well-known photosynthetic systems enable sustainable solar-to-chemical energy conversion. However, no evidence has yet emerged for the existence of a widespread geological light-harvesting system. This study reveals such a “photoelectric device”, where semiconducting Fe and Mn (oxyhydr)oxide mineral membrane is found to overlay vast expanses of natural rock/soil surfaces and exhibit highly responsive and stable photon-to-electron conversion. Our discovery may provide new insight supporting vital photon-induced redox chemistry on Earth’s surface via widespread Fe- and Mn-mineral membrane.