CHARACTERIZATION OF THE MERCER CLAY OF CENTRAL PENNSYLVANIA FOR LITHIUM AS A POTENTIAL FUTURE RESOURCE

The Mercer Clay, of the Pottsville group located in central Pennsylvania, is characterized by plastic clays with moderate aluminum (Al₂O₃ = 30-40%) content underlain by high alumina flint clays (Al₂O₃ = 38-50%) which have historically been used in refractories. Lithium (Li) has previously been found in the Mercer in concentrations up to 2100 ppm. However, the distribution of Li in the clay unit is poorly characterized. Abandoned mine highwalls near Morgan Run have been analyzed for Li, and further characterization of the Mercer under Clearfield County will help determine whether it may be a resource for Li.

Samples from the highwall, compositionally distinct sections of a continuous drill core through the Mercer, and three samples from the flint clay at the base of the Mercer section, were collected for analysis of Li and other critical minerals (CM). Each sample was prepared for inductively coupled plasma mass spectroscopy (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Combustion to remove organic material and quadruple acid digestion was used for ICP-MS and lithium metaborate fusion for ICP-AES. Successive leaching experiments were conducted using ammonium hydroxide and acetic acid to separate surface-adsorbed ions, salts, and carbonates from structurally bound components in silicate minerals. The minerals in the clay unit were then identified using X-ray Diffraction (XRD).

Lithium concentrations up to 1300 ppm were found in the highwall just beneath the Mercer coal and in the flint clays at the base of the unit. High Li concentrations are found in the high-Al flint clays, though equally high Li concentrations have been found in organic-rich plastic clays with lower alumina. Leaching experiments show that 99% of the lithium is structurally bound in silicate minerals, not adsorbed on clay surfaces or held in carbonates. Chlorite-group minerals identified in the Mercer clays by XRD are the most likely host of lithium.

As we move toward a balanced energy profile and away from dependence on coal and natural gas, we will see an increase in demand for Li and other CM necessary for the production of renewable energy alternatives. To increase the United States’ stability, it is necessary to locate domestic sources of CM.