Pentlandite

Pentlandite is a pale bronze-yellow sulfide mineral ((Fe,Ni)₉S₈) made of iron, nickel and sulfur. It is one of the important sources of mined nickel. Crystals are seen rarely; it is mostly found as grains or flame-shaped intergrowths inside pyrrhotite. It forms in ultramafic and mafic igneous rocks, separating out as the magma cools. Its name comes from the Irish naturalist Joseph Barclay Pentland, who first noted the mineral.

Pentlandite is a nickel ore, mined at large deposits such as Sudbury, Norilsk and Kambalda. Pentlandite ore often also contains copper (chalcopyrite), cobalt and platinum-group metals, which are won together. According to the USGS, the leading uses of primary nickel in the USA are alloys and steels, electroplating, and catalysts and chemicals; stainless and alloy steels and nickel-containing alloys make up more than 85 per cent of consumption.

It has a hardness of 3.5–4 on the Mohs scale. A fingernail cannot scratch it, but a steel knife can. Its specific gravity is 4.6–5 g/cm³.

Its best-known localities: Sudbury Basin, Ontario, Canada; Voisey’s Bay, Labrador, Canada; Norilsk, Krasnoyarsk Krai, Russia; Kambalda, Western Australia; Rustenburg district, Merensky Reef, Bushveld Complex, South Africa.

Known localities in Türkiye: Maden (Ergani copper deposit, Anayatak), Elazığ, Türkiye; Dağküplü ophiolite, Eskişehir, Türkiye.

Its colour is pale bronze-yellow and its lustre metallic; its streak (powder colour) is pale bronze-brown. It does not react to a magnet, whereas the pyrrhotite it is often intergrown with does. This is the easiest way to tell them apart. It is soft (hardness 3.5–4) and brittle; it breaks with a shell-like curved surface (conchoidal). It has no cleavage but parts along the {111} plane in octahedral shapes. It looks very much like other bronze-yellow sulfides such as pyrrhotite, chalcopyrite and pyrite. A magnet (pyrrhotite is attracted, pentlandite is not), hardness, streak and the octahedral parting give clues; but nickel content can only be proved by laboratory analysis.

On the “street to diamond” value ladder it sits on step 4 of nine: Fine dimension stone and ore. Pentlandite is valued as an ore: the nickel content of the ore (its grade), the size of the deposit, by-products won with it such as copper, cobalt and platinum-group metals, the cost of mining and processing, and demand for nickel are what count. Because most nickel goes into stainless steel and alloys, demand moves with those industries. For collectors, pentlandite rarely forms fine crystals; its worth depends on the size and distinctness of the specimen and on a famous locality such as Sudbury.

Pentlandite contains nickel (about 19–34 per cent by weight in the samples in the Handbook of Mineralogy). Nickel is the most common cause of contact allergy: for people with nickel allergy it is wise to avoid prolonged skin contact with this mineral. Nickel compounds are dangerous when their dust is breathed in: the International Agency for Research on Cancer (IARC) found sufficient evidence of carcinogenicity in humans for the mixtures of nickel sulfides and oxides met in nickel refining, and classes nickel compounds as carcinogenic to humans (Group 1). So do not grind or sand pentlandite or breathe its dust. Do not put it in your mouth: swallowing nickel compounds can cause nausea, vomiting, diarrhoea and headache. Do not heat it; irritating sulfur dioxide gas is released. Do not put it in water to make “gem water” or an “elixir”. Wash your hands after handling it, and keep specimens away from children and pets.