File:Sphalerite in ironstone concretion (lower Pottsville Group, Lower Pennsylvanian; Irish Ridge East roadcut, near Trinway, Ohio, USA) 4 (38697193542).jpg

Sphalerite in concretion from the Pennsylvanian of Ohio, USA.

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 about 5400 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.

Sphalerite is a somewhat common zinc sulfide mineral (ZnS). It has a metallic to submetallic to resinous to adamantine luster. Many metals can substitute for the zinc, such as iron, cadmium, and manganese. Sphalerite almost always has some iron in it, so a better chemical formula would be (Zn,Fe)S. Sphalerite has a wide color range, depending principally on iron content. Pure to almost pure sphalerite is whitish to greenish. With increasing iron content, sphalerite becomes yellowish to brownish to blackish. One variety of sphalerite has a strikingly intense dark red color (ruby sphalerite). It's streak color also varies with iron content from whitish to pale yellowish to brownish. Sphalerite is also distinctive in being moderately heavy for its size and having six different planes of cleavage.

Sphalerite is the most important zinc ore mineral. Zinc produced from sphalerite is used for many purposes, including mixing with copper to produce brass, rust protection of iron & steel, and for making modern American pennies (although the cost of making each zinc penny is >1¢).

The rock shown here is an irregularly-shaped ironstone concretion that was hosted in gray nonmarine shales. This is a crack surface showing the interior of the concretion. The darker-colored material is sphalerite. The specimen comes from the Pottsville Group, a Pennsylvanian-aged, cyclothemic succession in eastern Ohio that contains nonmarine shales, marine shales, siltstones, sandstones, coals, marine limestones, and chert ("flint"). The lower Pottsville dates to the late Early Pennsylvanian. The upper part dates to the early Middle Pennsylvanian. The Lower-Middle Pennsylvanian boundary is apparently somewhere near the Boggs Member (?).

Stratigraphy: lower Pottsville Group, upper Lower Pennsylvanian

Locality: Irish Ridge East Outcrop - roadcut along the eastbound entrance ramp from Rt. 60 to Rt. 16 (southeastern portion of Rt. 16-Rt. 60 interchange), north-northwest of the towns of Trinway & Dresden, northern Cass Township, northwestern Muskingum County, eastern Ohio, USA (~40° 09’ 13.56” North latitude, ~82° 01’ 29.37” West longitude)

Photo gallery of sphalerite: