Formation of halotrichite in the South Kerala Sedimentary Basin, SW India: Implications for Martian paleo-environmental studies

Halotrichite [FeAl₂(SO₄)₄·22(H₂O)] is a rare secondary sulfate mineral with its occurrence confined within the sulfide weathering zones. In the South Kerala Sedimentary Basin (SKSB) of SW India, halotrichite, here reported for the first time, is associated with an organic matter (OM)-rich carbonaceous clay layer. Field investigation revealed the prevalence of acid rock drainage (ARD) conditions similar to Martian analogue sites like Rio Tinto. The OM-layer is associated with pyrite forming substratum and Al-rich leachate associated with the Youngest Toba Tuff (YTT) cryptotephra layer. Oxidation of these units results in the formation of halotrichite, which is temporally restricted only to the dry season when the water table lowers and the OM-layer is exposed to air. X-Ray Diffraction (XRD) results for halotrichite show the presence of Al and Fe(II) sulfates. Energy Dispersive X-ray Spectroscopy (EDS) ruled out the existence of pickeringite, the Mg end-member of halotrichite-pickeringite series. XRD results for the OM-layer indicate the presence of kaolinite, quartz, goethite, and lepidocrocite. Hyperspectral analysis of the clay samples confirms that halotrichite is associated with goethite, lepidocrocite, kaolinite, and smectite. Fourier Transform Infrared Spectroscopy (FTIR) analysis revealed the major constituent of the clay sample as kaolinite with traces of quartz, smectite, and OM. Raman spectrum of halotrichite shows the symmetric stretching vibration of SO₄²⁻ bonded to Fe²⁺. Association of halotrichite with Fe, Al, and S-rich minerals is also inferred by the study of Raman spectrum of the host clay. Co-existence of halotrichite, goethite, lepidocrocite, and phyllosilicates in the study area is similar to the mineral assemblage found along the Rio Tinto River. On Mars, halotrichite is associated with the layered sulfate deposits, such as those in Valles Marineris and Meridiani Planum, and is categorized as a polyhydrated sulfate. Hence, this study on the halotrichite mineralization in the SKSB can supplement the attempts on deciphering the deposition and formation environment of similar mineralization on Mars.