Three kinds of slags occur in the waste dump of pyrometallurgical slags produced during reworking of lateritic ores of Ni in Szklary, Lower Silesia, southwestern Poland. Slags 1 and 2 contain 34–44 wt.% SiO2, and slag 2 is enriched in Ca and impoverished in Fe relative to slag 1; slag 3 is characterized by extreme Ca content ( ca. 50 wt.% CaO). The slags consist of silicate glass, synthetic equivalents of clinopyroxenes (diopside, hedenbergite), melilite, olivines (forsterite, fayalite) and subordinate spinel (chromite), sulfides (pyrrhotite, pentlandite, heazlewoodite, digenite) and intermetallic compounds (awaruite, bronze, metallic Fe and Cu). One type of slag contains significant amounts of the potentially toxic elements (PTE) Co (>380 ppm), Cr (>6400 ppm), Ni (>4000 ppm) and Zn (>352 ppm). The phase assemblages and textures in the Szklary slags are similar to those in other pyrometallurgical slags produced during reworking of silicate and sulfide ores, but they contain fewer sulfides. Although the slags have been exposed to atmospheric conditions for 30–80 years, those occurring in the dump are not affected by weathering, and small vitreous fragments of slag occurring in nearby agricultural fields have only thin (<100 μ m) crusts due to weathering. Nanometric inclusions of sulfides and metallic alloys are embedded in the silicate glass of vitreous slags. Some PTE are concentrated in these silicates: diopside is enriched in Cr (up to 2.3 wt.% Cr2O3), forsterite in Ni (up to 1.7 wt.% NiO), and melilite in Zn (up to 0.7 wt.% ZnO), but their reactivity was found to be limited in the alkaline soils surrounding Szklary.