Semi-Quantitative Analyses of Ferromanganese Nodules from the Pacific Ocean Using Synchrotron X-ray Powder Diffraction

The geochemistry and mineralogy of Mn nodules offer crucial insights into the origins, environmental changes, and distribution of abyssal resources. However, the conventional laboratory X-ray diffractometer, usually employed for semi-quantitative analysis of mineral composition in Mn nodules, often fails to sufficiently detect minor phases due to beam flux limitations and high background signals. In this study, we investigated differences in manganate composition, even when comprising around 1% of the phase fraction, in two manganese nodules (KC-8 and KODOS-10) using high-resolution synchrotron X-ray diffraction. The Mn/Fe ratios of KC-8 and KODOS-10 were 1.32 and 6.24, respectively, indicating that KC-8 and KODOS-10 were predominantly formed in hydrogenetic and diagenetic environments. Both samples contained quartz, vernadite, buserite, and feldspar. Todorokite and illite were exclusively observed in KODOS-10. In KC-8, the phase fractions of vernadite and buserite among manganates ranged from 94(5)%-100(4)% and 6(1)%-0%, respectively. However, in KODOS-10, the fractions of vernadite, buserite, and todorokite ranged from 47(1)%-56(2)%, 33.6(4)%-40.1(3)%, and 10(3)%-16.3(8)%, respectively.