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Cobaltite & erythrite (Frontier Mine, Ontario, Canada) | by James St. John
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Cobaltite & erythrite (Frontier Mine, Ontario, Canada)

Cobaltite and erythrite from Ontario, Canada. (~2.4 cm across at its widest)


Silvery-gray = cobaltite

Pinkish-purple = erythrite


A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.


The sulfide minerals contain one or more sulfide anions (S-2). The sulfides are usually considered together with the arsenide minerals, the sulfarsenide minerals, and the telluride minerals. Many sulfides are economically significant, as they occur commonly in ores. The metals that combine with S-2 are mainly Fe, Cu, Ni, Ag, etc. Most sulfides have a metallic luster, are moderately soft, and are noticeably heavy for their size. These minerals will not form in the presence of free oxygen. Under an oxygen-rich atmosphere, sulfide minerals tend to chemically weather to various oxide and hydroxide minerals.


Cobaltite is an important cobalt ore mineral having the formula (Co,Fe)AsS - cobalt iron arsenic sulfide. It's essentially arsenopyrite with cobalt. Cobaltite has a metallic luster, a bright silvery color, a dark gray streak, and is moderately hard (H = 5.5). It can form cubic and pyritohedral crystals (the same crystal forms as pyrite), but cobaltite has cleavage, unlike pyrite. It frequently occurs as finely granular masses mixed with other minerals (see below). Cobaltite occurs in some hydrothermal veins, and in some contact metamorphic rocks.


Shown above is a nice piece of cobaltite from Ontario's Frontier Mine. Mineralization at the Frontier Mine occurs as veins (= mineralized fault zones and veins close to faults) cutting the Proterozoic-aged Nipissing Diabase Sill and Archean-aged Keewatin rocks (principally basalts cut by lamprophyre dikes).


The pinkish-purplish material is the scarce mineral erythrite (Co3(AsO4)2·8H2O - hydrous cobalt arsenate), which forms as an alteration product of cobalt-bearing minerals such as cobaltite.


Locality: Frontier Mine, "town" of Silver Centre, South Lorrain Township, Cobalt-Gowganda region, Timiskaming District, southeastern Ontario, southeastern Canada


Photo gallery of cobaltite:


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Taken on April 4, 2016