Lab 3: Minerals II - Sulfates


Sulfate minerals consist of the sulfate ion group, SO 4 , combined with another element. Sulfur behaves in a manner similar to the way the carbonate ion group bonds with other elements. Gypsum is called a hydrous calcium sulfate mineral, because water molecules are locked into the crystal structure. Barite is simply barium sulfate. Here are the elements involved in these sulfide minerals:



Elements in sulfates


Gypsum (Selenite variety)


Chemical formula: CaSO 4 . 2H 2 O (calcium sulfate)


Structure: Gypsum is in the monoclinic crystal system. There are three varieties of gypsum: satin spar forms in seams in sedimentary layers and has a fibrous structure; alabaster is a massive form with a "sugary" appearance; selenite is a clear, crystalline form. A similar mineral, anhydrite, forms when water is driven out of the crystal structure of gypsum by heat (Anhydrite is CaSO 4 ).


Hammer behavior: The cleavage in selenite is interesting. There is one good direction, leading to the main "slab" or flat shape, but there are two other directions, at 66 and 114 degrees, and running perpendicular to the flat shape. The other two cleavage directions may be seen in a selenite crystal as "almost breaks," or "cracks" at those angles.


Color: Selenite is clear, the other varieties tend to be white.


Streak: white


Luster: For selenite, vitreous.


Diaphaneity: For selenite, transparent to translucent


Hardness: 2


Occurrence: Gypsum is most common in evaporite deposits, notably of Permian and Cretaceous age. As with halite, gypsum forms when water is evaporated to become highly saline. Gypsum precipitates (grows crystals) before halite and other salts start to grow.


Use: A main use of gypsum is in the manufacture of wallboard (sheetrock), which is made by squirting a slurry of gypsum and water between two big sheets of paper. The gypsum hardens within a few minutes, and makes for a smooth, relatively inexpensive wall material for houses. Gypsum is also common in soil and weathering zones, especially in arid areas where water evaporates in the soil zone.




Chemical formula: BaSO 4 (barium sulfate)


Structure: The crystal structure of barite is orthorhombic.


Hammer behavior: Barite


Color: white or colorless usually, also red, brown, yellow, blue


Streak: white


Luster: vitreous, when massive specimens of small crystals


Diaphaneity: transparent to translucent


Hardness: 3


Occurrence: Barite is often found in mineral veins associated with lead deposits, around hot volcanic vents, and in association with hematite.


Use: Barite is the principle source of barium. It is used in the manufacture of paper. And, barite is crushed as added to drilling mud, which increases the weight of fluid in the borehole of a drilling oil well, so that unexpected pressures, if encountered, will be balanced.