Siderite-rhodochrosite (Fe,Mn,Ca,Mg)CO3, a solid solution of four major components from the hydrothermal sediments of Atlantis II Deep, Red Sea, was studied in samples collected from the SW basin and the SW passage.The carbonates appear as clusters of euhedral to subhedral, 2-3 μm crystals, and form a thin layer, slightly admixed with clay minerals, short-range ordered round particles (singerite), goethite, and halite. Although siderite was found to be the major component within the carbonate layers, solid solutions of siderite, rhodochrosite, calcite, and magnesite are common. Due to isomorphous substitution, unit cell parameters of the carbonates studied are shifted towards higher or lower values in solid solution with calcite or magnesite, respectively.Most of the siderite samples appear as Mn-siderite with ratios of Mn/Fe 0.17-0.21, with only one pure siderite sample. Calcium and magnesium substitute to a lesser extent, having ratios of Ca/Fe 0.04-0.17 and Mg/Fe 0-0.09 with variation within and between the different crystals. Calcium, iron, and magnesium substitute for Mn in rhodochrosite with the ratios of Ca/Mn = 0.12-0.16, Fe/Mn = 0.05-0.27, and Mg/Mn = 0.02-0.03.Apparently, precipitation of thin layers of Mn-siderite occurred via direct crystallization of the inflowing brine as it was injected into the uppermost part of the brine system. Additional Mn to form Mn-siderite, oligonite, and rhodochrosite could have precipitated through the whole brine system mainly depending on Fe2+ injection into the brine system.The δ13C values reflect mixture between lowermost convective layer (LCL) brine and transition zone (TZ) zones, whereas δ18O values are mainly derived from crystallization temperatures.