Crystal structure of dehydrated ulexite, NaCaB5O6(OH)6·3H2O

Abstract Ulexite, a key borate mineral exploited in various industrial applications, undergoes thermal transformations. Here, we report the determination of the crystal structure of dehydrated ulexite, NaCaB5O6(OH)6·3H2O, for the first time, using the single-crystal X-ray diffraction technique. Thermal analysis and synchrotron powder diffraction reveal that ulexite, NaCaB5O6(OH)6·5H2O, loses two H2O molecules at 100 °C, causing the formation of a stable three-hydrated phase that retains the fundamental building block (FBB) observed in ulexite. The dehydrated ulexite is triclinic with space group P1¯ and lattice parameters a = 8.686(4) Å, b = 10.973(5) Å, c = 6.709(3) Å, α = 105.463(5)°, β = 107.518(5)°, γ = 79.833(5)°, and V = 584.5(5) Å3. A distinctive rearrangement of the Na coordination environment from the original six- to seven-coordinated geometry is observed, whereas the Ca atom retains an eight-coordinated geometry. The Na-O and Ca-O bond distances range from 2.340 to 2.852 Å and 2.379 to 2.646 Å, respectively. These polyhedra form edge-sharing chains that interconnect in two-dimensional sheets on the b-c plane. The FBB of the dehydrated ulexite comprises double three-membered rings consisting of a central BO4 tetrahedron, a BO2(OH)2 tetrahedron, and BO2(OH) triangle sharing corners, described as 2Δ3□: –. The average B-O bond distances of two Bφ3 triangles (φ: O or OH) are 1.363 and 1.368 Å, whereas those of three Bφ4 tetrahedra are 1.475, 1.477, and 1.474 Å. The dehydrated ulexite is stable up to 160 °C. Our findings not only resolve longstanding ambiguities regarding the nature of the so-called one-hydrated phase but also suggest the possibility that dehydrated ulexite naturally emerges as a stable hydrated borate mineral. New insights into the thermal behaviors and dehydration mechanisms of hydrated borate minerals have implications for both natural mineralization processes and industrial applications.