Simulated and measured scattering matrices of feldspar are compared at wavelength to evaluate the accuracy of using spheroids to model single-scattering properties of natural irregular mineral particles (feldspar). The results of the comparison show that the model of spheroids, whose scattering properties are calculated using a T-matrix method, is usable and far superior to spheres in describing scattering by feldspar particles not much larger than the wavelength. Simulations with prolate ellipsoids are better in agreement with measurements than those with oblates, and an average over a wide range of axis ratios is better than any single value. The refractive index affects the angle dependence of scattering rather weakly, as long as its real and imaginary parts are varied in reasonable ranges. These findings imply that an inversion of a refractive index or shape information would be quite difficult. On the other hand, they indicate that the T-matrix method is easier to apply to different problems, as less information about the target particles is required.