Magnetic frustration in low-dimensional substructures of hulsite <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ni</mml:mi><mml:mrow><mml:mn>5.15</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Sn</mml:mi><mml:mrow><mml:mn>0.85</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>BO</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>

This paper presents an extensive study of the structural, magnetic, and thermodynamic properties of the hulsite ${\mathrm{Ni}}_{5.15}{\mathrm{Sn}}_{0.85}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$. The crystal structure of the hulsite has two planar substructures formed by Ni and Sn atoms: one with rectangular configuration and the other with a triangular arrangement. These substructures are linked by the boron ions and by Ni in another site closer to the rectangular arrangement, resulting in a quasi-two-dimensional character. Thus, this system literally adds a new dimension to the study of oxyborates. Our results point to a complex magnetic behavior consistent with these substructures. The planes with rectangular arrangement form a complex magnetic ordering at 180 K (one of the highest magnetic transitions among the oxyborates). The other subsystem, formed by Ni atoms located in a two-dimensional triangular lattice, does not order down to temperatures as low as 3 K. The experimental results suggest a spin-liquid behavior for this subsystem. The magnetic moments of the ions between these planes also freeze at low temperatures. The two magnetic planes coexist as independent subsystems down to the lowest temperatures of our experiments.