The Stability of Manganese Oxides Under Laser Irradiation During Raman Analyses: II. Layer Structures

Manganese oxides (MnOx) are primary components of marine polymetallic deposits, which represent strategic reservoirs for critical metals essential for the transition to sustainable energy (including lithium, cobalt, and tellurium). Due to their nature (intergrown micro- to nano-sized and often poorly crystalline minerals), MnOx are challenging to investigate. Consequently, the application of methods for fast, highly-resolved and possibly remote and non-destructive identification of MnOx is critical in several disciplines, from ore geology to materials science. Micro-Raman spectroscopy is commonly used for this purpose. However, MnOx are sensitive to laser irradiation and, therefore, can undergo significant modification during analysis. For this reason, it is essential to establish a proper analytical protocol for their identification. In this study, we present our results on the behavior of the most common MnOx with layer structures under irradiation by a 532-nm laser, with intensity ranging from 23 μW/μm2 to 36.8 mW/μm2, continuing our recent work on dense and channel MnOx. Our results show that birnessite, ranciéite, chalcophanite, vernadite, and asbolane are stable up to 391 mW/μm2, while lithiophorite endures up to 2.5 mW/μm2; however, its stability is significantly reduced when other metal cations substitute Mn and Al. Under higher laser power, all MnOx transform into a spinel-type phase such as hausmannite, jacobsite, Mn-rich hetaerolite, or galaxite, depending on the chemical composition of the starting material. Based on our data, we propose an innovative analytical protocol for robust phase identification by analyzing Raman scattering at varying laser power, from the starting material to its final degradation product.