English: Acanthite from the Precambrian of Morocco. (9 millimeters across along the base)

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 5600 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.

Acanthite (a.k.a. argentite) is a scarce silver sulfide mineral - Ag2S. On fresh surfaces, acanthite has a metallic silvery-gray appearance. On less fresh surfaces, it has a dull metallic luster and a dark gray to black-colored tarnish. Acanthite lacks cleavage, has a black streak, is fairly soft (H = 2 to 2.5), and is fairly heavy for its size. Acanthite is the most important silver ore mineral.

Geologic context & age: Imiter Silver Deposit, mineralization at ~550 Ma (near-latest Neoproterozoic)

Locality: Imiter Mine, northern side of Saghro Mountain, Ouarzazate Province, Anti-Atlas Mountains, central Morocco

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Photo gallery of acanthite:

www.mindat.org/gallery.php?min=10