Chromogenic and Chromotropic Mechanisms of Color-Changing Fluorite from the Huanggangliang Area, Inner Mongolia

The Huanggangliang area in Inner Mongolia is a major fluorite and polymetallic ore district in China. Its skarn-type deposit yields rare color-change fluorite, yet the coloration and photochromic mechanisms remain poorly studied. Five faceted samples were characterized by conventional gemological tests, EDXRF, UV-Vis, Raman, PL, TL, and FTIR to determine their composition, spectral features, and defects. The results indicate two distinct color-forming mechanisms. Group I is dominated by f-f transitions of rare-earth elements with color-center synergy, showing a strong gray-blue to blue-violet color change and a characteristic absorption peak at ~580 nm in the UV-Vis spectrum. Group II is dominated by high-concentration irradiated color centers. Its deep blue-violet color and weak color change are attributed to colloidal induced by long-term endogenous α, β, and γ irradiation from radioactive Th. The lack of additional Raman peaks and a broad UV-Vis absorption peak at ~595 nm, along with multiple UV color-center peaks, weak fluorescence, and thermoluminescence, all confirm the presence of high-concentration electron traps. This study establishes the composition–spectrum–color relationship, reveals the control of ore-fluid evolution on mechanism differentiation, and provides a scientific basis for identifying and analyzing color-change fluorite.