Formation of Melanterite From Marcasite: Insights Into Martian Transient Wet Conditions

Abstract This study reports the first occurrence of melanterite (Fe 2+ SO 4 ·7H 2 O), a secondary hydrous sulfate mineral, in Peninsular India and examines its formation through the oxidative weathering of marcasite associated with lignite in Neyveli, Tamil Nadu. The marcasite was exposed to normal atmospheric conditions, and melanterite was formed within 23 days via oxidative weathering. Melanterite occurs as a yellow, prismatic to fibrous, translucent crystal, either as individual fibers or clusters. Though melanterite is mineralogically characterized through different techniques like X‐ray diffraction, scanning electron microscopy‐energy dispersive X‐ray spectroscopy, and electron probe microanalysis (EPMA), the results obtained from EPMA (Fe 0.839 Cu 0.000355 Zn 0.000466 Ni 0.000545 ) SO 4 ·7H 2 O closely match the composition of standard melanterite. Fourier Transform Infrared spectroscopy identified key vibrational peaks for sulfates (450–480 cm −1 ) and water molecules (1,544–1,650 cm −1 ), while laser Raman spectroscopy provided evidence for the characteristic vibration of melanterite at 976 cm −1 . Hyperspectral analysis also discerned the characteristic Fe 2+ peak (0.45–0.9 μm) of melanterite. A spectrum, comparable to that of melanterite, obtained through Compact Reconnaissance Imaging Spectrometer for Mars data from the Ceti Mensa region on Mars, where it is associated with a polysulfate deposit. Since melanterite forms under acidic and oxidizing conditions in low moisture, its occurrence can provide insights into the environmental conditions under which sulfate forms. Thus, this study can provide insights into studying transient wet conditions on Mars.