Daughter crystals of sassolite, crystalline boric acid (H3BO3), are discovered for the first time in gas-liquid inclusions in minerals from the Mika and Amazonitovaya pegmatite veins in the Kukurt gem cluster (Central Pamirs). The crystals of sassolite are rounded, tabular, less frequently, idiomorphic, with low refractive indices and high birefringence. The Raman spectrum of sassolite has an intense line near 880 cm−1 and a weaker one near 499 cm−1. Sassolite crystals in inclusions of types 1 and 2 in quartz from a miarolitic cavity of the Mika vein occur in associations with daughter crystals of halite and Cs-bearing sylvite. In inclusions of type 3 in quartz and tourmaline from the Mika vein and in quartz and adularia from the Amazonitovaya vein sassolite is the only daughter mineral. The study of sassolite-bearing inclusions has shown that the miarolitic cavities and near-cavity complexes of the Mika and Amazonitovaya veins formed with active participation of concentrated solutions of boric acid (6.7–14.8 and 5.9–10.0 wt.% H3BO3, respectively) having various proportions of CO2 and chlorides and fluorides of K, Li, Cs, Fe, and Mn. The presence of daughter sassolite suggests boron migration in the form of H3BO3 under hydrothermal conditions. The data obtained are of great interest for reconstructing the conditions of mineralization in both granitic pegmatites and hydrothermal systems, in which boron played an active role.

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