Laurite

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Formula: RuS₂
Sulphide of ruthenium, pyrite group, forms a series with erlichmanite
Crystal System: Isometric
Specific gravity: 6.43 measured, 6.39 calculated
Hardness: 7½
Streak: Dark grey
Colour: Iron-black
Luminescence: Nonfluorescent under UV
Common impurities: Os,Rh,Ir,Fe
Environments

Igneous environments

Laurite occurs in ultramafic complexes and in placer deposits. Associated minerals include cooperite, braggite, sperrylite, other minerals of the platinum group elements and chromite (HOM).

Localities

At Kraubath an der Mur, Leoben District, Styria, Austria, and at Hochgrößen, Oppenberg, Rottenmann, Liezen District, Styria, Austria, the dunite–harzburgite massifs were interpreted as a strongly metamorphosed ophiolite sequence. Unaltered podiform chromitite occurs in both localities. At Kraubath the platinum group mineral assemblage in the unaltered podiform ores is dominated by laurite. Sperrylite is the most abundant platinum group mineral in the altered chromitite, whereas minerals of the laurite–erlichmanite series are subordinate. At Kraubath, osmium-bearing platinum group minerals (laurite, erlichmanite, ruarsite and an osmium-iridium alloy) occur as single grains and as complex polyphase assemblages. At Hochgrössen, laurite and an osmium-iridium alloy are present as solitary grains only (CM 41.331–352).

At the type locality, Tanah Laut, Banjar Regency, South Kalimantan Province, Indonesia, grains of laurite have been found. Inclusions are rare, but one grain contains calcium-aluminium amphibole inclusions, and another contains an inclusion of chalcopyrite + bornite + pentlandite + heazlewoodite that is considered to have formed by a two-stage process of exsolution and crystallisation from a once homogeneous iron - copper - nickel sulphide melt (MM 68.2.353-368).

At the Stillwater Complex, Montana, USA, chromitite found in layered intrusions and ophiolite complexes is generally enriched in platinum group elements, especially iridium, osmium and ruthenium, and the chromitite is usually poor in base metal sulphides. The most common platinum group mineral is laurite, predominantly enclosed in chromite grains. In a few cases the laurite is accompanied by rarer and smaller platinum group minerals, including malanite and platinum-palladium sulphides. Interstitial to the chromite grains the assemblage is dominantly platinum-, palladium- and rhodium- bearing, including palladium-lead, platinum-palladium tellurides, sperrylite, platarsite, minor laurite, and one grain of palladium-germanium. The base metal sulphides that were not included in chromite exsolved to form pentlandite, pyrrhotite and platinum group minerals (CM 55.1.121-144).

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