New data on the Gongo Soco palladium-oxygenated compounds are presented for the first time since the work of Johnson and Lampadius (1837). A variety of PdO species is documented, always in association with Cu, Fe and Mn, and includes Pd(OH)2 or PdO.H2O, and phases with metal excess in relation to oxygen, interpreted as metastable. Their remarkable occurrence in ‘ouro preto’ (black gold) and jacutinga (auriferous itabirite/iron ore) is the result of mertieite-II alteration. It involves leaching of Sb and As, limited removal of Pd, and incorporation of Cu, Mn and Hg into PdO species. The locus of Hg-bearing PdO species in microfractures parallel to the enclosing gold-mineralized shear band suggests at least one low-temperature hydrothermal generation. The presence of a halite crystal attached to gold testifies to the involvement of Cl-saturated hydrothermal fluids in the oxidizing environment of jacutinga formation.

Idiomorphic crystals (up to 3.5 mm) of ferroan platinum, cooperite and mertieite-II were found in a heavy-mineral concentrate from stream sediments of the Darya river in the Aldan Shield, Russia. Pt-Fe crystals display cubic and thin platy habits; occasionally they are twinned. The chemical composition ranges from Pt2.64Fe1.00 to Pt2.88Fe1.00 with Os, Ru, Ir, Rh and Pd below the analytical detection limit of the electron microprobe. X-ray diffractometry of Pt-Fe crystals suggests a F-centred cubic lattice, characteristic of ferroan platinum. Some of the ferroan platinum crystals have large (about 100 μm wide) cooperite overgrowth rims or are covered by a Au-Ag alloy. Cooperite also occurs as large euhedral crystals (up to 3 mm across, partly twinned). Crystals of mertieite-II are speckled with μm-sized (2–5 μm) inclusions of sperrylite and intergrown with minerals of cooperite-braggite solid solution, Pt-Pd-Hg alloy, keithconnite and Au-Ag alloy. Fractures along crystallographic planes of the mertieite-II crystals are filled with Pd-Pt-Fe-Sb-As-Hg-Te-Bi-bearing oxides. The coarse-grained PGM from the Darya have a geological setting similar to the Kondyor PGE placer 75 km to the northeast and are probably related to clinopyroxenite-hornblende-magnetite units of Alaskan-/Uralian-type intrusions.

Coarse-grained mertieite-II [Pd8(Sb,As)3] crystals (up to 2.5 mm across) with inclusions of sperrylite and intergrown with keithconnite [Pd3(Te,Bi)], cooperite-braggite, a Pt-Pd-Hg alloy, a (Pd,Pt)9Te phase and Au-Ag alloy occur in alluvial sediments in the Darya river, Aldan Shield, Russia. The primary PGM assemblage underwent alteration with formation of a very fine-grained mixture of Pd-bearing oxides and hydrated oxides. The Pd oxide/hydrated oxide aggregates (up to 100 μm wide and ~1 mm long) occur along crystallographic planes, on grain boundaries and fractures, occasionally intergrown with goethite. The Pd-O-bearing compounds formed after (1) mertieite-II (Sb content reaches 23 wt.%), (2) keithconnite (variably enriched in Te, up to 7 wt.% and Bi, up to 6 wt.%) and (3) potarite (with high Hg content, up to 35 wt.%). The variably oxidized and hydrated PGM assemblage can be attributed to supergene alteration and small-scale element redistribution under cool climatic conditions with a limited degree of leaching.

The crystal structure of the mineral mertieite-II from the Kaarreoja River, Inari commune, Finnish Lapland, Finland, was refined to R1 = 0.0222 (I) and 0.0228 (II) on the basis of X-ray diffraction data collected from two single crystals. The mineral is trigonal, space group is $R\bar 3c$. The unit-cell parameters for the two crystals are determined as: a = 7.5172(3), c = 43.037(2) Å, V = 2106.1(2) Å3 (I); a = 7.5135(4), c = 43.003(3) Å, V = 2102.4(3) Å3 (II) with Z = 12. The occupancies of the Sb and As position in the structure were refined according to the Sb:As ratio of each crystal. The position As1 (Wyckoff 6b) is completely filled by As atoms. Any excess of As is distributed together with Sb on structural position M1 (Wyckoff 12c). The crystal chemical formulae are defined as Pd8Sb1.5(Sb0.94As0.06)As0.5 (crystal I) and Pd8Sb1.5(Sb0.88As0.12)As0.5 (crystal II). As → Sb substitution of up to 4.50 wt% of As does not affect the main structural topology of mertieite-II. Mertieite-II, Pd8Sb2.5As0.5 and synthetic Pd8Sb3 are isotypic compounds. The crystal structures of synthetic Pd8Sb3 and natural mertieite-II, Pd8Sb2.5As0.5, can be derived from the hexagonal close packing by filling additional layers between the close-packed 36 layers. The structure consists of Sb and (Sb,As) triangular 36, Pd triangular 36 and Pd pentagonal-triangular 5.33 layers. Stacking of Sb-, (Sb,As)- and Pd-nets along the z axis caused extension of the unit cell. It contains a total of 36 layers: six Sb1-nets, six (M1,As1)-nets, 12 Pd1-nets and 12 (Pd1,Pd3,Pd4)-nets.