Abstract Members of the isostructural ludwigite-vonsenite series of anhydrous borates resemble the hydroxyl-bearing silicate ilvaite in habit, color, luster, density, and qualitative optical properties. Suitable means of recognizing and identifying these minerals are desirable, owing to their different economic, petrologic, and geo-chemical significance. We investigated two ludwigites, two vonsenites, and two ilvaites by chemical, spectrochemical, X-ray, and optical methods. In composition, the four borates range from ~(Mg 1.98 Fe +2 .02 )Fe +3 BO 5 to ~(Mg .05 Fe +2 1.95 )Fe +3 BO 5 . Aluminum varies directly with Mg, from 2.2 weight per cent in Mg-rich ludwigite to 0.8 weight per cent in Fe +2 -rich vonsenite. Tin ranges from < 0.004 per cent in Mg-rich ludwigite to 1.2 per cent in ludwigite from a tin-mining district. Unit-cell dimensions and cell volumes of the members vary somewhat irregularly, generally increasing as Fe +2 increases and Al decreases. Increase in Al seems to decrease the cell volume greatly in Mg-rich ludwigite. Indexed X-ray powder diffraction data are useful in estimating Fe +2 content of the members. Mg-rich ludwigite is translucent, with mean n = 1.866 and orange = 8.8 per cent. Opacity of the members increases markedly with Fe +2 content. Intermediate members are nearly opaque; the Fe +2 -rich vonsenite is completely so, with orange = 12.6 per cent. Reflectivity in air and oil varies directly with Fe +2 contentreflectivity in air varies inversely with wave length. Tin seems to increase the dispersion of the reflectivity. Bireflectance generally varies directly with wave length and Fe +2 content. Dispersion of the bireflectance is v > r for Mg-rich ludwigite, r » v for the rest. The two ilvaites are similar to each other in cation content and unit-cell dimensions. They contain ~6 weight per cent Mn; the average for 30 published analyses of ilvaite is ~3 per cent. In reflected light, the two ilvaites have similar but measurably different optical properties, possibly because of undetermined differences in OH, F, and CI content. For these ilvaites, orange = 7.6 and 7.7 per cent. The orientation X = c in transmitted light is consistent with Rp = Rc in reflected light. The ilvaites differ from the ludwigites and vonsenites in (1) crystallographic orientation of reflection-pleochroism and complex indicatrix of reflectivity, (2) effect of oil immersion on reflection-pleochroism, (3) reflectivity in air and oil, (4) bireflectance and its dispersion. Other physical properties, such as nonmagnetic character and behavior in transmitted light, are helpful in distinguishing ilvaite from most members of the ludwigite-vonsenite series, but X-ray powder data and optical properties in reflected light are generally more useful.