The Schwartz 'D' surface is another triply periodic minimal surface like the gyroid.. In this model, the bound is a sphere of radius 1.5 Pi . Like the gyroid, this surface also appears in nature, frequently in oil/water surfaces where the oil separates the water into two domains at the microscopic level. The equation for the surface is: Sin [x] Sin[ y] Sin[ z] + Sin[ x] Cos[ y] Cos[ z] + Cos[ x] Sin[ y] Cos[ z] + Cos[ x] Cos[ y] Sin[ z] = 0. This is a trig approximation to the truly minimal surface, which has a more complex equation.