The effect of solution chemistries and freezing temperatures on the morphology of cryogenic opal-A (COA): Implications for past climates on Mars

Abstract

Cryogenic opal-A (COA) is a type of hydrated amorphous silica that forms when silica-rich geothermal fluids erupt sub-aerially and are exposed to air temperatures below 0 °C. As the fluid cools and water-ice forms, the solution increases in ionic strength and reaches saturation of opal-A. This results in the precipitation of opal-A between water ice crystals. Natural COA is found embedded within frozen fluids near hydrothermal sources, and has been shown to preserve signs of biological activity. In this study, two solution chemistries (hydrochloric acid (HCl) and sulfuric acid (H2SO4)) and three freezing temperatures (−20, −80, −196 °C) were used to synthesize COA in the lab. SEM analyses showed that COA frozen at −20 °C in HCl solutions produced smooth silica branches similar to what was observed in previous studies. COA precipitated at lower freezing temperatures demonstrated progressively smaller particle sizes and unique morphologies such as halite dendrites within silica, desiccation cracks, and pores. COA made in H2SO4 solutions was more angular and fragmented than COA made in HCl solutions. Particle size also decreased with decreasing freezing temperature. COA textures have implications for terrestrial samples as well as future samples returned from Mars because they can provide information about past climate and fluid chemistry. Plain language summaryCryogenic opal-A (COA) is a hydrous silicate material that can form when geysers and hot springs erupt and are exposed to air temperatures below 0 °C. COA forms in brine veins that are between water ice crystals and takes the shape of the veins in which they form. COA has been shown to preserve signs of biological activity. In this study, two solution chemistries (hydrochloric acid (HCl) and sulfuric acid (H2SO4)) and three freezing temperatures (−20, −80, −196 °C) were used to form COA in the lab. Scanning electron microscope analyses showed that, in general, COA particle size decreased with decreasing freezing temperature. Additionally, COA made in HCl solutions were smooth and tubular whereas COA made in H2SO4 solution were angular and fragmented. COA textures have implications for terrestrial samples as well as future samples returned from Mars because they can provide information about past climate and fluid chemistry.