Summary: The arsenopalladinite concentrates from Itabira, Minas Gerais, Brazil, have been found to contain three arsenide-antimonides of palladium, namely arsenopalladinite, atheneïte, and isomertieite. The second and third of these are new minerals.

Arsenopalladinite, redefined, is Pd₅(As,Sb)₂ and triclinic with a 7·399, b 14·063, c 7·352 Å, α 92° 03′, β 118° 57′, γ 95° 54′. Z = 6. D_meas = 10·4, D_calc = 10sd46. In reflected light arsenopalladinite is white with a yellowish creamy hue. The mineral shows complex polysynthetic twinning and is strongly anisotropic. Reflectance measurements at 470, 546, 589, and 650 nm respectively gave: in air, 46·67–48·86, 49·97–52·90, 52·82–54·96, and 55·61–57·72 in oil, 32·30–35·07, 37·12–39·40, 38·97–41·32, and 40·28–43·07. VHN₁₀₀ 379–449, av. 407.

Atheneïte, (Pd, Hg)₃As, is hexagonal, space group P6/mmm and cell dimensions a 6⇒798, c 3·483 Å. The strongest lines of the powder pattern are 2·423 vvs (111) , 2·246 vs (201), 1·371 s (212), 1·302 s (302), 1·259 s (321). Z =2. D_calc = 10·16. In reflected light atheneïte is white with a faint bluish tint compared to arsenopalladinite. Anisotropy distinct. Untwinned. Reflectivities for the two grains examined are: in air, 470 nm 47·51–54·75, 47·43–51·18; 546 nm 50·79–58·01, 51·36–54·36; 589 nm 53·13–61·01, 53·24–55·86; 650 nm 55·94–63·13, 54·76–56·77; in oil, 470 nm 30·03–43·67, 33·46–37·31; 546 nm 33·42–47·75, 37·64–41·07; 589 nm 35·80–49·04, 39·40–42·24; 650 nm 38·25–50·49, 41·07–42·85. VHN₁₀₀ 419–442, av. 431.

Isomertieite, (Pd,Cu)₅(Sb,As)₂, is cubic, space group Fd3m, a 12·283 Å. The strongest lines of the powder pattern are 2·356 vs (333, 511), 2·167 vvs (440), 0·8599 s (10.10.2, 14.2.2), 0·8206 s (12.8.4), 0·7996 s (10.10.6, 14.6.2), 0·7881 s (999, 1.11.1, 13·7·5, 15·3·3), 0·7801 s (12.10.2, 14.6.4). Z = 16. D_calc = 10·33. In reflected light isomertieite is a pale yellow colour. One grain was isotropic, three others displayed weak anisotropy. Untwinned. Reflectance measurements at 470, 546, 589, and 650 nm gave respectively: in air, 44·74–46·46, 52·23–53·25, 55·05-57·49, 56·97–62·03; in oil, 31·04–31·40, 38·42–38·90, 40·80–42·16, and 42·91–45·63. VHN₁₀₀ 587–597, av. 592.

Quantitative colour values are also given, and the chemical and optical properties are compared with the related mineral, stibiopalladinite.