Gypsum

Mineral Name: Gypsum

Chemical Composition: CaSO4.2H2O, hydrous calcium sulfate

Color: Gray, white, brown, clear

Streak: White

Hardness: 2

Cleavage/Fracture: Gypsum’s cleavage is complex, there is only good cleavage in one direction and fair in two others that create thin, rhombic fragments.

Crystal Form: Gypsum appears mostly as a four sided crystal, otherwise known as tabular crystal. This is also identified as prismatic, acicular or bladed (8). Gypsum crystals and fibrous masses can also be curved resulting in shapes that look like “Ram Horns” (8). Crystals can also form fishtail or swallowtail twins. The twins are formed when there are two or more crystals who share a portion of the same crystal lattice. Also, gypsum forms crystals that tend to be large in size (8).

Luster: Pearly to resinous

Special Features: Gypsum can have thin crystals that can bend but they aren’t elastic so they can’t bend back on their own (1). On Moh’s scale of hardness, gypsum has a hardness of two. This means that it is a very soft mineral, soft enough to scratch with your fingernail. Some samples of gypsum are fluorescent and also have a very low thermal conductivity. Low thermal conductivity means that gypsum is not able to transport energy through its crystal at a very quick rate (1).

Varieties: There are four crystalline varieties of gypsum, which are satin spar, desert rose, gypsum flower, and selenite, all of which can sometimes be grouped together forming selenite.

Mineral Group: Gypsum is part of the sulfate group, which is a group of minerals that consist one or more metallic elements and the sulfate ion (SO42-). All sulfates are soft and transparent to translucent. They can be lightly colored and some are soluble in water. The hydrous sulfates are a group of sulfate minerals that contain water in the chemical structure. (8)

Environment: The environment in which gypsum can be found is in huge deposits of sedimentary rock. It can also be found in clay sedimentary deposits and in dry caves. Gypsum can also be found in oxidation zones of sulfide deposits. (8)

Associated Rock types: The most common rock type associated with gypsum is rock gypsum. You can also find gypsum with the minerals sulfur, calcite, dolomite, and anhydrite in layered sedimentary deposits (2).

Occurrence in North America: It can be seen in many places such as the Great Salt Plains in Oklahoma, the Mojave Desert in California, many counties located in Ohio, Utah, Texas and New York, as well as the Mammoth caves in Kentucky. (8)

Economic and Industrial Uses: Gypsum is a non-toxic mineral that can be used to help humans, animals and plants. Gypsum can be used in a variety of ways, one of those are as a food additive (FDA approved) for conditioning water in brewing beer, to control the color and taste of wine, and as a main ingredient in canned vegetables, flour, bread, ice-cream and cheese. Th gypsum crystalline powder is added to clear mineral deficiencies in the water that's being used for the different food products above.

Gypsum is also used as plaster and drywall. In order to do this, gypsum is heated to cause the water to evaporate, leaving behind white powder. Afterwards, water is poured into the powder and the gypsum becomes moldable. The material then gets sandwiched between paper boards and is compressed to give it strength. The boards are set through a machine that heats up to about 400 degrees fahrenheit, to remove the water. Once the gypsum dries, it hardens and maintains its shape. (10)

First Notable Identification: In the 18th century, French chemist Lavoisier began research on gypsum studying the chemical properties. In Paris large deposits of gypsum, otherwise known as “plaster of Paris”, were discovered. Plaster of Paris is gypsum that is heated to remote the water molecules from its composition: CaSO4.2H2O.(5) After this discovery the use of gypsum was more common.

How We Identified It: You can mainly identify gypsum by cleavage and hardness (2) as well as remembering that it looks similar to halite, although it doesn’t taste like salt.

Don’t Confuse It With: Halite. You can tell these minerals apart by licking both. Halite has a very salty taste and Gypsum doesn’t. Fear not, neither are toxic.