Correlation Between Crystalline Order, Micro-Morphology, and Thermal Stability in “Heijin” (Black Gold) Seal Stone from Changhua, China: A Pyrite-Bearing Dickite Aggregate

“Heijin” (the literal translation from Chinese being “Black Gold”) seal stone represents a unique variety of sulfur-rich, dickite-dominant jade, yet its mineralogical genesis and structural properties remain insufficiently characterized. This study utilizes a multi-analytical approach comprising polarized light microscopy, X-Ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), Scanning Electron Microscopy coupled with Energy-Dispersive X-Ray Spectroscopy (SEM-EDS), Electron Probe Microanalysis (EPMA), and Thermogravimetry and Differential Scanning Calorimetry (TG-DSC) to investigate the phase composition, crystalline order, and thermal evolution of this material. The results demonstrate that “Heijin” stone is primarily composed of highly ordered 2M₁ dickite with a Hinckley index (HI) ranging from 0.92 to 1.50. Its distinctive black appearance originates from the disseminated distribution of micrometer-scale pyrite, which is accompanied by trace amounts of svanbergite. This aluminum phosphate–sulfate (APS) mineral serves as a critical indicator of high sulfur fugacity and acidic hydrothermal alteration environments. Furthermore, a significant correlation exists between the crystalline order of dickite, its micro-morphology, and its thermal stability. Samples characterized by high crystallinity (HI ≈ 1.50) exhibit well-developed, euhedral book-like aggregates and elevated dehydroxylation temperatures (T_m ≈ 665 °C), whereas samples with lower crystalline order correspond to fragmented microstructures and reduced thermal stability. This research defines the mineralogical identity of “Heijin” stone and provides a scientific basis for employing thermal analysis to evaluate the crystalline quality of dickite-based jade materials.