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Genkinite
Genkinite is composed primarily of platinum, palladium and antimony. Based on the information from electron probe analyses of eight grains, the ideal formula of genkinite is derived to be (Pt,Pd)4Sb3. Unsuccessful attempts to synthesize Pt4Sb2 have suggested that genkinite does not occur in a binary phase of the Pt-Sb system. Instead genkinite is a ternary phase in the Pt-Pd-Sb system. This conclusion is based on the further successful synthesis of (Pt,Pd)3Sb2 with 19.3 weight percent Pd and (Pt,Pd)4Sb3 with 17.7 weight percent Pd. This experiment yielded result similar to the genkinite found in the Onv erwacht mines. Both of these examinations show the amount of Pt to be greater than Pd which is why the empirical formula is written as Pt3PdSb3. The composition of genkinite differs slightly from Onverwacht mine and Shetland Islands. The composition from the Onverwacht mine corresponds to (Pt2.17Pd0.81Rh0.61Ni0.32Cu0.04)sum=3.95(Sb2.81As0.11Bi0.08)sum=3.00 with Z=8, while the composition from the Shetland Islands is (Pt2.04Rh0.76Pd0.62Ni0.55)sum=3.97Sb3.03. . These differences can be attributed mainly to the environments in which they formed as the Onverwacht mine is chromite and silica rich while the Shetland Islands are an ophiolite environment with a chromite rich lithology.

Table 1: Electron Microprobe Analysis of Genkinite listed as wt% element and atomic properties ) . Structure The crystal structure of genkinite has not been fully determined. After a precession photograph, the mineral was determined to be tetragonal and belong to the 422 point group . An indexed Gandolfi pattern was preferred to get additional cell data information. It was determined that a = 7.736(1) Å and c = 24.161(2) Å . Table 2 is an X-ray powder pattern of one of the crystals. The information the analysis provided was enough to assign P41212, P4122, P4322, P43212, P42212, or P4222 as the six possible space groups of genkinite .  A more thorough X-ray crystal structure study will have to be done to determine which of these six is correct. The x-ray pattern of genkinite from the Onverwacht mine in South Africa was determined to be 2.265 (10) Å, 3.020 (9) Å, 1.934 (6) Å, 1.910 (5) Å, 0.9043 (5b) Å, 0.9025 (5b) Å, 2.146 (4) Å.

Table 2: X-Ray Powder Data for Genkinite.

Physical Properties Genkinite is an opaque, pale brown mineral with a small measure of yellow coloration. The mineral also has a metallic luster. With the information from the electron microprobe analysis, and assuming Z=8, the calculated density of genkinite is 9.256 g/m3 and a Vickers hardness of 603-677 (25g load). The mineral has a weak bireflectance in oil but not in air with a white light, which suggests it is isotropic. The anisotropic properties of Genkinite are moderate to strong; from gray to extinction (Cabri et al., 1977b). The reflectance can be found from around 52% at 470nm to around 56% at 650nm. Image 1 below is a genkinite inclusion in Isoferroplatinum. Image 1: Scanning electron micrograph image with a 60 micron field of view.

Geologic Occurrence Platinum-group minerals, in this case, occur on the rim of chromite rich rocks in an interstitial silicate matrix as shown in the schematic diagram in Figure 1 (below). The image on the right shows the mineral map of genkinite from the Harold’s Grave location in Scotland. Genkinite is represented by [(Pt, Rh, Pd, Ni)4Sb3] and is always found in association with other platinum bearing platinum group minerals. At the Onverwacht mines, genkinite was found as a lamellar inclusion to a Pt-Fe alloy which can be seen in Image 1.

Figure 1. Schematic diagram of PGM occurrence (Left) and a Genkinite mineral map (right).

The ophiolitic environment with a chromite-rich lithology at Harold’s Grave on the Shetland Islands accounts for the fourth occurrence of genkinite. Ophiolitic environments occur when a section of the oceanic crust and upper mantle have been uplifted and exposed above sea level. These environments are traditionally thought to contain iridium, osmium and ruthenium. The finding of genkinite, sperrylite (PtAs2), mertieite (Pd11(Sb,As)4), hongshiite (PtCu) and potarite (PdHg) has altered this traditional thought. In all instances genkinite has only been found to occur as irregular grains and ranges between 5 to 165 microns in length. It is closely associated with sperrylite and platarsite, because of its platinum content, which can be seen in Image 2. Image 2: Enclosure of (1) Pt-Fe alloy, (2) Platarsite, (3) Genkinite, (4) Chromite crystals in a platinum-group mineral.

Who in the world On July 9th, 2010, Alexandr Dimitrievich Genkin died in Moscow. Genkin was the pioneer in the study of the platinum-group elements in sulfides. His contribution to ore mineralogy and the mineralogy and genesis of the Noril’sk ores continue to be of major importance today. Dr. A.D Genkin was a Soviet mineralogist who furnished both mineralogy and geochemistry with important discoveries in the platinum-group elements. It is because of this, that in 1977 the mineral Genkinite was been named in his honor. His work in the Onverwacht platinum deposit and the discovery of the sulfarsenide irarsite with ruthenian hollingworthite was very important in the continued discoveries that were to follow in the platinum-group minerals.