Mineralogical, Gemological Characteristics and Petrogenesis of High-Quality Maw-Sit-Sit Jade from the Myanmar Jade Belt

Maw-sit-sit jade resembles kosmochlor-jadeitite in appearance and is spatially associated with it in the Myanmar Jade Belt. However, the mineral composition, microstructure, and petrogenesis of this type of jade remain unclear. To address this gap, this study investigated high-quality Maw-sit-sit jade using a range of analytical techniques, including conventional gemological tests, infrared spectroscopy, petrographic observations, electron probe microanalysis (EPMA), and backscattered electron (BSE) imaging. Results show that Maw-sit-sit jade primarily consists of albite and chromium-omphacite, with minor amphibole (eckermannite and richterite). Jadeite and relict chromite are absent in the studied samples. Its high albite content gives it lower refractive index (RI: 1.55–1.56) and specific gravity (SG: 2.69–2.73) compared to kosmochlor-jadeitite and jadeite jade. Additionally, Maw-sit-sit jade exhibits punctate or banded fluorescence under ultraviolet (UV) light, distinguishing it from kosmochlor-jadeitite and jadeite jade (both inert). Petrographically, euhedral albite fills interstices between early-formed Cr-omphacite and eckermannite, which is textural evidence of its late-stage origin. Eckermannite and Cr-omphacite occur as enclosed grains with embayed boundaries and dissolution pores, indicating they experienced mechanical disruption and chemical dissolution during subsequent geological processes. Petrogenetically, Maw-sit-sit jade (defined as “Cr-omphacite-albitite”) forms via a two-stage process: (1) Under high-pressure/low-temperature (HP/LT) conditions in the subduction zone, Na-Al-Si-rich fluids metasomatize chromite-bearing serpentinite protoliths, generating an early assemblage of jadeite, Cr-omphacite and amphiboles; (2) During subsequent plate exhumation and decompression, jadeite underwent retrograde metamorphism under low-pressure/low-temperature (LP/LT) conditions involving residual Na-Al-Si fluids, resulting in the formation of albite. This process led to the replacement of early-formed minerals by euhedral albite, ultimately generating the Ab+Cr-Omp+Eck symplectic texture. This study elucidates the mineralogical, gemological identity and petrogenesis of high-quality Maw-sit-sit jade, advancing our understanding of fluid evolution within a subduction zone.