7-12 Maximizing Fossil Yield by Minimizing Concentrate: A chemical Process to Remove Diagenetic Selenite from Screen Wash Concentrate from the Upper Triassic Downs Quarry (Adamanian) of Arizona

Session: Undergraduate Research, Part I (Posters)

Presenting Author:

Authors:

Abstract:

Screenwashing fossiliferous sediments is one of the best ways to recover tiny fossils (e.g., microfossils and microvertebrates). However, not all rocks are amenable to breakdown in water alone. We are working with concentrate collected in 1983 by the Field Museum from the Upper Triassic (Adamanian) Placeras/Downs’ quarry in Arizona, arguably the most productive Triassic locality in North America. We have >50kg of screenwash concentrate that includes abundant selenite. Through diverse protocols, we tried to optimize the selenite dissolution, with minimal risk to the fossils. 

Visual analysis indicated that the concentrate contains ~37% selenite (CaSO₄), so removing it would reduce volume (and thus picking time) by ~35%. We developed three protocols for screen washing 100g batches of concentrate in DI H₂O (control), 5% NaCl, 10% NaCl, and 5% K₂CO₃ solution, each for 3-4 wash and dry cycles. After multiple tests of each protocol, the K₂CO₃ was clearly superior, removing almost 50% of the concentrate after 4 cycles compared to ~20% for the other washes. We suspect that all treatments reduced ~20% of the concentrate that was clay minerals and the K₂CO₃ removed another ~30% of the selenite. The K₂CO₃  treatment also bleached the unfossiliferous concentrate white, in contrast to the black fossils.Thus, we expect picking to go more than twice as quickly.

The K₂CO₃ process is a simple displacement reaction—the K₂CO₃ dissolves, causing the selenite to go into solution. The reaction forms potassium sulfate as an aqueous product and calcium carbonate as a precipitate—K₂CO₃(aq)+CaSO₄(s)->K₂CO₃(aq)+CaCO₃(s). The process requires a dilute acetic acid (vinegar) rinse between washes, where the CaCO₃ and acetic acid react and produce aqueous calcium acetate and aqueous bicarbonate ions. CaCO₃(s)+CH₃COOH(aq)->Ca(CHCOO)₂(aq)+HCO-₃(aq)

We scaled up the K₂CO₃ protocol process to 1000g batches to process all 50 kg of concentrate. We plan “experiments” with student volunteers who participate in “Finding Fossils on Friday” (FFF) to see if picking treated concentrate is indeed more efficient, and how many washes work best for recovery. K₂CO₃ is about $27.20 per 500g container; if we can find a “sweet spot” in the amount of washes in comparison to recovery, this would minimize cost and maximize recovery by reducing picking time.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.