Sulfur

The brimstone of myth and legend, sulfur’s gifts run the gamut from the healing power of many medicines to the destructive power of gunpowder. Although not an important rock-forming mineral, sulfur is still a crucial resource for human societies. Sulfur is a native element as well as a mineral. Its crystal form consists of transparent to translucent crystals that have an unmistakable deep yellow color.  In its massive form, the color is usually a pale yellow, but may sometimes be red or gray if impurities are present. Moisture in the air reacts with sulfur to release small amounts of hydrogen sulfide (H2S), giving sulfur a distinctive odor similar to rotten eggs. Since hydrogen sulfide is poisonous though, rely on color rather than smell to identify sulfur samples, even though samples do not give off high levels of the gas. Its smell, combustible nature, and association with volcanic activity gave rise to sulfur’s role in literature as the brimstone of underworld and hellish settings.

Description and Identifying Characteristics

Its yellow color and odor are sulfur’s most distinctive features. Samples of sulfur are very soft (1.5 to 2.5 on the Mohs scale of hardness) and have a low specific gravity, so they feel ‘light’ for their size. Sulfur also melts at a relatively low temperature (108o C) and becomes brittle when heated. It is such a poor conductor of heat that simply holding a sulfur sample in your hand can cause its surface to expand and break away, while placing a sample on a sunlit windowsill is a sure way to destroy it.

Seldom the dominant mineral present, sulfur usually occurs as an accessory mineral associated with past or present volcanic activity, or from the alteration of pyrite and other sulfide minerals deposits. However, sulfur deposits large enough to be of commercial value almost always form from the alteration of gypsum and anhydrite deposits. Both are sulfate minerals that form as seawater evaporates. When gypsum and anhydrite are deeply buried, bacteria can alter these deposits to form sulfur. Natural oil and gas deposits associated with gypsum rock units can also hold large volumes of hydrogen sulfide gas. Within the past few decades, these gas deposits have replaced the mineral sulfur as our main source of the element.

Sulfur has been used since prehistoric times and is the ‘brimstone’ mentioned in the Bible and other ancient records. The word sulfur is Latin for ‘burning stone’ and sulfur lives up to its name. Alchemists once considered sulfur to be the essential element of all combustion. Romans used it as an incendiary weapon, long before its use in gunpowder led to a number of technological and historical revolutions. Greeks and Romans recognized many of sulfur’s other benefits. For over 4000 years, it has been used as a fumigant to control or drive off insects and for medicinal salves.

Modern uses of sulfur have expanded to include the vulcanization (hardening) of natural rubber, water treatment systems, textiles and paper production. Some sulfur is also used in the manufacture of steel and aluminum, but over 90% of the sulfur mined by modern society is used to produce sulfuric acid (H2SO4). Sulfuric acid in turn is used in the manufacture of fertilizers and a host of chemical products. Fertilizer production alone accounts for roughly half of our present sulfur use, but sulfur also provides us with matches, dyes, and battery acid. With its distinctive color, you might expect sulfur to be used in the manufacture of yellow dyes and pigments, but it is too reactive to be a good dye. Sulfur is used in dye processes, but primarily to bond pigments to textiles and papers so the colors are more permanent.

Sulfur has also provided some unexpected economic benefits. It was the search for sulfur deposits beneath the Gulf Coast seafloor that led to the discovery of immense oil and gas fields in salt domes off the coast of Louisiana and Texas.

When sulfur-bearing compounds are burned, sulfur combines with oxygen to form sulfur dioxide (SO2).  Sulfur dioxide released into the atmosphere from the burning of fossil fuels, such as gas, petroleum, and coal, is presently a leading cause of air pollution. In the atmosphere, sulfur dioxide can combine with moisture to produce ‘acid rain’, precipitation with low amounts of sulfuric acid. Acid rain can damage building stones and monuments, cause a decline in natural lake systems and, in some areas, even pose a significant risk to human health.

Sulfur is almost always accompanied by some H2S, which is a poisonous gas. Fortunately its distinctive smell usually serves to warn people of its presence.

There are no significant natural occurrences of sulfur in the Upper Midwest region, although trace amounts occur in Wisconsin’s lead mining district. These traces formed from the alteration of pyrite in sulfide-rich vein deposits.

Perhaps the most obvious reminder of sulfur in the Upper Midwest is the distinctive odor emitted by pulp and paper mills across central and northern Wisconsin where sulfur is used to reduce wood pulp for paper production.

Because of its distinctive color and odor, there are no common minerals that are easily mistaken for sulfur. Carnotite, a relatively rare mineral, is the only natural mineral that might be confused with sulfur.

Carnotite:

Carnotite is an important, though rare, uranium ore. Its yellow color and powdery habit mimic that of sulfur, but you can usually distinguish the two by their associated host rock. Carnotite typically forms in sandstones from the alteration of uranium-bearing minerals. Sulfur is usually associated with gypsum and carbonate rocks.