COMPARING LAB AND NATURALLY GROWN GYPSUM GROWTH MORPHOLOGIES

Gypsum, CaSO₄* 2H₂O, is created through evaporation of water that, when supersaturated, leaves behind molecules that bond to each other to nucleate and precipitate a gypsum crystal. Gypsum crystals grow extending from a nucleation point. Gypsum can grow quickly from hypersaline brines and also over time from high pressure solutions within rock fractures (Clarey, 2012). This project asks whether unique features of natural gypsum samples can be replicated in lab grown gypsum morphologies to help discern particular depositional environments. I compare growth morphologies of gypsum sampled from two very different environments and aim to replicate natural growth morphologies through lab experiments. Modern brine samples studied were from Bonaire, a municipality of the Netherlands in the Caribbean Sea. Gypsum forms abundantly along Kaminda Lac Road and the Cargill Salt Works concentrating pans, where it grows as a byproduct of the halite production process (Antczak, 2018). Gypsum grown in Bonaire was mainly found to have clusters of small crystals around which larger crystals grow outward from the center in a columnar habit. Individual crystals were platy and prismatic with much evidence of twinning. Both the Triassic Chugwater Formation, Owl Creek Mountains, WY, and the Cretaceous Mowry Shale, Sheep Mountain, WY, exhibit ancient gypsum that has grown within sedimentary rock formations. Vein gypsum crystals grew as lamellae perpendicular to the plane of the fracture surface.

Lab experiments found that open glass vessels allowing for the most evaporation produced the largest crystal growth. Experiments use a sous vide to heat a supersaturated solution to a constant temperature of 100 °F. Pressurized experiments took place in a pressure cooker with solution inside. Lab experiments produced gypsum crystals up to 5 mm of mostly platy morphologies, but some prismatic crystals. Small crystal clusters have been found in some lab experiments. Morphologies of gypsum produced in a lab setting were compared to natural gypsum samples. Scanning Electron Microscope BSE/SE merged images were generated and stitched together to compare morphologies on the micrometer to millimeter scale.