The 13th Reynolds Cup 2026 is an international biennial round robin competition designed to assess the accuracy and precision of quantitative mineral analysis, with a strong focus on clay mineralogy. Organized by CSIRO Mineral Resources, the competition distributed three blind, complex mineral mixtures representing iron ore, lithium-hosted, and critical minerals deposits. Each sample contained multiple crystalline, poorly crystalline, amorphous, and layer silicate phases, requiring participants to apply robust analytical methods, most commonly X-ray diffraction. The event attracted 124 registrations from 35 countries, with 108 results submitted. Rankings were determined by cumulative analytical bias across all three samples. The top five placings were announced at the Annual Meeting of the Clay Minerals Society Annual Meeting, held in Provo, Utah, USA on the 10th of July 2026. The top 3 places received plaques, and the winner, with the lowest bias score was also presented with the Reynolds Cup trophy. All other participant scores were kept anonymous. The competition highlights global capability in quantitative mineralogy, supports methodological benchmarking, and continues the Reynolds Cup tradition of advancing best practice in mineral analysis.