PETROGRAPHIC DIFFERENCES BETWEEN ACID AND NEUTRAL HALITES

Halites formed in acid and neutral saline lakes in Western Australia show distinguishable petrographic differences. Acid saline lakes maintain a range of pH from 1.5 – 4 and are hypersaline (up to 280 °/°° total dissolved solids). Waters are primarily Na-Cl-Mg-SO4-rich with variable high concentrations of Al, Fe, Si, and Br. Petrographically, halite from acid lakes generally exhibit more coloration. For example, yellow halite forms where Al-rich acid waters are trapped as fluid inclusions in halite. Orange halite forms by hematitic solid inclusions. Iron oxide and sulfate daughter crystals are also more common within acid fluid inclusions. Acid fluid inclusion freezing/melting behaviors show distinct characteristics that differ from neutral fluid inclusion behaviors. Acid fluid inclusion freezing/melting behaviors tend to have more complex metastable phases at specific melting temperatures than the neutral fluid inclusions, and develop a discrete, definable, fuzzy border on inclusion rims. The acid inclusions also have higher eutectic temperatures, higher and wider temperature range of hydrohalite rims, and more complex final melting behaviors of various crystals. Acid halite contains “hairy blobs”, clusters of organic material and sulfate crystals, which are not found in halite from neutral lakes. This distinct assemblage of features characteristic of modern acid lake halites may serve as informal criteria for the recognition of past acid saline lake evaporites.