The refractive indices for light vibrating along the a, b, and c cell edges -- symbols na, nb, and nc - were measured for 20 andalusite-kanonaite solid solutions ranging from relatively pure andalusite (Al₂SiO₅) to about 62 mole percent kanonaite (MnAlSi0₅). Of the 20 crystals, 8 contained significant Fe³⁺ in addition to Mn³⁺ and, in the literature, were called viridines. Linear regressions were performed on the data using, successively, na, nb, and nc for 589.3 nm as the dependent variables and, as independent variables, among other choices, the total sum of the weight percents for Mn₂O₃ plus Fe₂O₃. The resultant models na = 1.6444 + 0.00225 (Mn₂O₃ + Fe₂O₃) r²=0.99 nb = 1.6399 + 0.0036 (Mn₂O₃ + Fe₂O₃) r²=0.98 nc = 1.6325 + 0.0066 (Mn₂O₃ + Fe₂O₃) r²=0.98 confirm and explain Wulfing's (1917) observation that, relative to the {110} cleavage, andalusite is length-fast whereas viridine is length-slow. Thus, the equations for na and nc intersect when (Mn₂O₃ + Fe₂O₃) equals 2.74 weight percent. Consequently for (Mn₂O₃ + Fe₂O₃) 2.74 the optic orientation is a=Z, b=Y, c=X (as in andalusite) whereas for (Mn₂O₃ + Fe₂O₃) 2.74 the orientation is a=X, b=Y, c=Z (as for viridines and kanonaites). Andalusites at or near the 2.74 weight percent intersection should likely be isotropic or nearly so. Thus, for wavelength 589.3 nm, their indices would likely be na = nc = 1.6506 and nb=1.6498. Even at wavelengths 486.1 or 656.3 nm their maximum birefringence would be little m0re than 0.0012. To the writer's knowledge, andalusites containing (ca.) 2.74 weight percent (Mn₂O₃ + Fe₂O₃) have yet to be described. Either they are scarce or absent in nature or else they escaped recognition as andalusites because of their near-isotropism. Linear regressions performed on the optical data for andalusite only (viridines and kanonaite excluded) yield equations that indicate that, for pure andalusite for λ = 589.3: α = nc = 1.6325(3); β = nb = 1.6385(3); γ = na = 1.6434(3); 2Vx = 84 degrees. For most andalusites, 2V varies little from 84 degrees but does appear to increase with Mn content. For pure andalusite the cell edges, volume, and density are predicted to be a = 7.7917(7)A, b = 7.8961(2)A, c = 5.5536(6)A, V = 341.68A³, and d = 3.149gm/cc³. / M.S.