A new clay play: the first halloysite-hosted ion-adsorption REE discovery in the United States

In December 2025, Ionic Mineral Technologies announced the discovery of a rare earth element (REE) and critical-mineral occurrence named Silicon Ridge in the Soldiers Pass area of central Utah, described as a halloysite-hosted ion-adsorption clay (IAC) system. Because halloysite-hosted REE occurrences have not previously been documented in the United States, the U.S. Geological Survey is conducting independent sampling and geologic evaluation to assess the proposed enrichment and to develop a process-based model for REE emplacement and preservation. Our initial conceptual model for the occurrence involves the deposition of felsic volcanic ash in a lacustrine environment with localized hydrothermal activity, followed by alteration to halloysite-rich horizons composed of high-surface-area tubular particles. Subsequent near-surface weathering and shallow groundwater flow may have mobilized REE over short distances and promoted low-level, spatially diffuse adsorption onto halloysite surfaces, consistent with ion-adsorption-style behavior. Preservation of any adsorbed REE would be favored by attenuation of groundwater flow following deposition of overlying units, reducing permeability and limiting continued flushing. Laboratory-based geochemical analyses are being conducted to evaluate REE concentrations, distributions, and exchangeability, and to test this proposed enrichment model. Independent of final REE concentrations, the framework developed for Soldiers Pass provides a process-based model for evaluating the geologic conditions under which halloysite-bearing systems may host REEs in the United States.