Lab 3: Minerals II - Sulfides

 

Sulfide minerals consist of the element sulfur combined with another element. Note that there is no oxygen present in the sulfides - they are without oxygen. Here are the elements involved in these sulfide minerals:

 

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Elements in sulfides

 

Pyrite

 

Chemical formula: FeS 2 (iron sulfide)

 

Structure: Pyrite crystalline structure is within the isometric crystal system, and grows in cubic crystals. The structure of pyrite is different from its polymorph, marcasite (which has the same chemical formula).

 

Hammer behavior: When struck with a hammer pyrite fractures unevenly, or with slight conchoidal fracture.

 

Color: brass yellow, but tarnishes to brown

 

Streak: dark gray to black, sometimes with green or brown tones

 

Luster: metallic

 

Diaphaneity: opaque

 

Hardness: 6.5 to 6

 

Occurrence: Pyrite is often found in sedimentary rocks such as dark muds or plant fossil rich clay, where there is little available oxygen to combine with iron. Don't confuse it with gold; its nickname is "fool's gold." It can be associated with gold, however. Pyrite is also found in quartz veins and some metamorphic rocks.

 

Use: Pyrite has been used in the paper industry for making sulfur dioxide and in the making of sulfuric acid.

 

Chalcopyrite

 

Chemical formula: CuFeS 2 (copper iron sulfide)

 

Structure: Chalcopyrite is in the tetragonal crystal system.

 

Hammer behavior: Chalcopyrite fractures with slight conchoidal fracture and is brittle.

 

Color: golden yellow, but tarnishes to show purple, and is sometimes called "peacock ore."

 

Streak: dark gray

 

Luster: metallic, dull

 

Diaphaneity: opaque

 

Hardness: 3.5 to 4

 

Occurrence: Chalcopyrite is found with and may be confused with pyrite, but is found more commonly in metal ore deposits.

 

Use: Chalcopyrite is an important ore of copper.

 

Galena

 

Chemical formula: PbS (lead sulfide)

 

Structure: Galena is in the isometric crystal system, often with cubic, and sometimes octohedral crystals.

 

Hammer behavior: Galena shows cubic cleavage (three directions of weakness, at 90 degrees), which can make it difficult to distinguish between cubic crystals and cubic cleavage faces.

 

Color: silvery gray, may tarnish to duller gray

 

Streak: gray, to dark gray

 

Luster: metallic

 

Diaphaneity: opaque

 

Hardness: 2.5

 

Occurrence: Galena is found in lead-sulfur ore deposits, many of which form in limestone (carbonate rock), where metamorphism, with the action of chemical change, concentrates lead-bearing minerals in contact zones. A good example of this occurrence is the Lead Belt of Missouri, for which galena is the state mineral.

 

Use: Galena is a main ore of lead.

 

Special Property: The presence of lead makes galena very heavy. Specific gravity is a number representing the density of a substance relative to water. Many minerals have a specific gravity in the range of 2 to 3, and other metallic minerals have specific gravities in the 4 to 5 range, but galena has a specific gravity of about 7.5. That means galena is about 7.5 times "heavier" than an equivalent volume of water.

 

Sphalerite

 

Chemical formula: ZnS (zinc sulfide)

 

Structure: Sphalerite is in the cubic crystal system.

 

Hammer behavior: There are six planes of weakness in sphalerite, leading to hammer behavior that is most striking. When you move a specimen of sphalerite back and forth in the light, you will see many cleavage faces, seemingly at random angles, flashing back at you. This cleavage on planes "every which way" makes sphalerite distinctive.

 

Color: brown to yellow, black

 

Streak: white to yellow brown

 

Luster: resinous, submetallic

 

Diaphaneity: opaque

 

Hardness: 3.5 to 4

 

Occurrence: Sphalerite is commonly found with sulfur-rich ore deposits, along with galena and other similar minerals.

 

Use: Sphalerite is an important ore of zinc.