Sulfide Minerals

01
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Bornite

Copper iron sulfide
Sulfide Mineral Pictures. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

The sulfide minerals represent higher temperatures and a slightly deeper setting than the sulfate minerals, which reflect the oxygen-rich environment near the Earth's surface. Sulfides occur as primary accessory minerals in many different igneous rocks and in deep hydrothermal deposits that are closely related to igneous intrusions. Sulfides also occur in metamorphic rocks where sulfate minerals are broken down by heat and pressure, and in sedimentary rocks where they are formed by the action of sulfate-reducing bacteria. The sulfide mineral specimens you see in rock shops come from the deep levels of mines, and most display a metallic luster.

Bornite (Cu5FeS4)is one of the lesser copper ore minerals, but its color makes it highly collectible. (more below)

Bornite stands out for the amazing metallic blue-green color it turns after exposure to the air. That gives bornite the nickname peacock ore. Bornite has a Mohs hardness of 3 and a dark gray streak.

Copper sulfides are a closely related mineral group, and they often occur together. In this bornite specimen are also bits of golden metallic chalcopyrite (CuFeS2) and areas of dark-gray chalcocite (Cu2S). The white matrix is calcite. I'm guessing that the green, mealy-looking mineral is sphalerite (ZnS), but don't quote me.

02
of 09

Chalcopyrite

Copper iron sulfide
Sulfide Mineral Pictures. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Chalcopyrite, CuFeS2, is the most important ore mineral of copper. (more below)

Chalcopyrite (KAL-co-PIE-rite) usually occurs in massive form, like this specimen, rather than in crystals, but its crystals are unusual among the sulfides in having a shape like a four-sided pyramid (technically they are scalenohedra). It has a Mohs hardness of 3.5 to 4, a metallic luster, a greenish black streak and a golden color that is commonly tarnished in various hues (though not the brilliant blue of bornite). Chalcopyrite is softer and yellower than pyrite, more brittle than gold. It is often mixed with pyrite.

Chalcopyrite may have various amounts of silver in place of the copper, gallium or indium in place of the iron, and selenium in place of the sulfur. Thus these metals are all byproducts of copper production.

 

03
of 09

Cinnabar

Mercury sulfide
Sulfide Mineral Pictures. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Cinnabar, mercury sulfide (HgS), is the principal ore of mercury. (more below)

Cinnabar is very dense, 8.1 times as dense as water, has a distinctive red streak and has hardness 2.5, barely scratchable by the fingernail. There are very few minerals that might be confused with cinnabar, but realgar is softer and cuprite is harder.

Cinnabar is deposited near the Earth's surface from hot solutions that have risen from bodies of magma far below. This crystalline crust, about 3 centimeters long, comes from Lake County, California, a volcanic area where mercury was mined until recently. Learn more about the geology of mercury here.

 

04
of 09

Galena

Lead sulfide
Sulfide Mineral Pictures. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)

Galena is lead sulfide, PbS, and is the most important ore of lead. (more below)

Galena is a soft mineral of Mohs hardness of 2.5, a dark-gray streak and a high density, around 7.5 times that of water. Sometimes galena is bluish gray, but mostly it's straight gray.

Galena has a strong cubic cleavage that is apparent even in massive specimens. Its luster is very bright and metallic. Good pieces of this striking mineral are available in any rock shop and in occurrences around the world. This galena specimen is from the Sullivan mine in Kimberley, British Columbia.

Galena forms in low- and medium-temperature ore veins, along with other sulfide minerals, carbonate minerals, and quartz. These can be found in igneous or sedimentary rocks. It often contains silver as an impurity, and silver is an important byproduct of the lead industry.

 

05
of 09

Marcasite

Iron sulfide (orthorhombic)
Sulfide Mineral Pictures. Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Marcasite is iron sulfide or FeS2, the same as pyrite, but with a different crystal structure. (more below)

Marcasite forms at relatively low temperatures in chalk rocks as well as in hydrothermal veins that also host zinc and lead minerals. It doesn't form the cubes or pyritohedrons typical of pyrite, instead forming groups of spearhead-shaped twin crystals also called cockscomb aggregates. When it has a radiating habit, it forms "dollars," crusts and round nodules like this, made of radiating thin crystals. It has a lighter brass color than pyrite on a fresh face, but it tarnishes darker than pyrite, and its streak is gray whereas pyrite may have a greenish-black streak.

Marcasite tends to be unstable, often disintegrating as its decomposition creates sulfuric acid.

 

06
of 09

Metacinnabar

High-temperature mercury ore
Sulfide Mineral Pictures From the Mount Diablo Mine, California. Photo (c) 2011 Andrew Alden, licensed to About.com (fair use policy)

Metacinnabar is mercury sulfide (HgS), like cinnabar, but it takes a different crystal form and is stable at temperatures above 600°C (or when zinc is present). It is metallic gray and forms blocky crystals.

07
of 09

Molybdenite

Molybdenum sulfide
Sulfide Mineral Pictures. Photo courtesy Aangelo via Wikimedia Commons

Molybdenite is molybdenum sulfide or MoS2, the primary source of molybdenum metal. (more below)

Molybdenite (mo-LIB-denite) is the only mineral that might be confused with graphite. It's dark, it's very soft (Mohs hardness 1 to 1.5) with a greasy feel, and it forms hexagonal crystals like graphite. It even leaves black marks on paper like graphite. But its color is lighter and more metallic, its mica-like cleavage flakes are flexible, and you may see a glimpse of blue or purple between its cleavage flakes.

Molybdenum is necessary for life in trace amounts, because some vital enzymes require an atom of molybdenum to fix nitrogen to build proteins. It's a star player in the new biogeochemical discipline called metallomics.

 

08
of 09

Pyrite

Iron sulfide (cubic)
Sulfide Mineral Pictures. Photo (c) 2009 Andrew Alden, licensed to About.com (fair use policy)

Pyrite, iron sulfide (FeS2), is a common mineral in many rocks. Geochemically speaking, pyrite is the most important sulfur-containing mineral. (more below)

Pyrite occurs in this specimen in relatively large grains associated with quartz and milky-blue feldspar. Pyrite has a Mohs hardness of 6, a brass-yellow color and a greenish black streak.

Pyrite resembles gold slightly, but gold is much heavier and much softer, and it never shows the broken faces that you see in these grains. Only a fool would mistake it for gold, which is why pyrite is also known as fool's gold. Still, it's pretty, it's an important geochemical indicator, and in some places pyrite really does include silver and gold as a contaminant.

Pyrite "dollars" with a radiating habit are often found for sale at rock shows. They are nodules of pyrite crystals that grew between layers of shale or coal.

Pyrite also readily forms crystals, either cubic or the 12-sided forms called pyritohedrons. And blocky pyrite crystals are commonly found in slate and phyllite.

 

09
of 09

Sphalerite

Zinc sulfide
Sulfide Mineral Pictures. Photo courtesy Karel Jakubec via Wikimedia Commons

Sphalerite (SFAL-erite) is zinc sulfide (ZnS) and the foremost ore of zinc. (more below)

Most often sphalerite is reddish-brown, but it can range from black to (in rare cases) clear. Dark specimens can appear somewhat metallic in luster, but otherwise its luster can be described as resinous or adamantine. Its Mohs hardness is 3.5 to 4. It commonly occurs as tetrahedral crystals or cubes as well as in granular or massive form.

Sphalerite can be found in many ore veins of sulfide minerals, commonly associated with galena and pyrite. Miners call sphalerite "jack," "blackjack," or "zinc blende." Its impurities of gallium, indium and cadmium make sphalerite a major ore of those metals.

Sphalerite has some interesting properties. It has excellent dodecahedral cleavage, which means that with careful hammer work you can chip it into nice 12-sided pieces. Some specimens fluoresce with an orange hue in ultraviolet light; these also display triboluminescence, emitting orange flashes when stroked with a knife.