Gypsum vs Anhydrite: Difference and Comparison

Gypsum and anhydrite minerals are quite well known, especially among collectors, making them a very popular minerals. Both are forms of calcium sulfate in a way.

The two have many differences, which are explained in detail below. Both these minerals have been used as examples in textbooks, and their characteristics have been highlighted.

Key Takeaways

  1. Gypsum contains water molecules in its crystal structure, while anhydrite lacks water molecules.
  2. Gypsum is softer than anhydrite, with a Mohs hardness of 2, while anhydrite has a hardness of 3 to 3.5.
  3. Gypsum produces plaster, drywall, and fertilizer, while anhydrite is used for cement production and drying.

Gypsum vs Anhydrite

The difference between gypsum and anhydrite is that Gypsum is a mineral that is aqueous and contains 2 water molecules, while anhydrite, as it is known, is a mineral that is anhydrous and does not contain water molecules. The chemical formula of gypsum is (CaSO4 2H2O). On the other hand, the chemical formula of anhydrite is (CaSO4).

Gypsum vs Anhydrite
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The positive molecule of water in the gypsum attracts the negative molecule of oxygen. This is because hydrogen bonding will be stronger only if the temperature remains cool.

Gypsum becomes unstable when heat destroys hydrogen bonds and removes water from hydration. Gypsum is a very soft mineral and is a form of calcium sulfate.

Anhydrite is an orthorhombic mineral. It does not react with hydrochloric acid.

It is a hard mineral. It is a hard crystal with a hardness density of 3.0.

If we look at its hardness rating, then it is 3.5. It is considered a type of rare mineral.

It is also known as hydrous calcium sulfate.

Comparison Table

Parameters of comparisonGypsumAnhydrite
Definition     Gypsum is a mineral that is aqueous and contains 2 water molecules.Anhydrite, as it is known, is an anhydrous mineral and does not contain water molecules.
Chemical formula The chemical formula of gypsum is (CaSO4 2H2O). The chemical formula of anhydrite is (CaSO4).
Physical property Gypsum is softer.Anhydrite is a hard crystal. 
Structural formIt has a monoclinic structural form.It has an orthorhombic structural form. 
Density    The density of gypsum is about 2.33g/cc.The density of anhydrite is 3.2g/cc. 

What is Gypsum?

Gypsum is a type of mineral that is soft and is the basic form of calcium sulfate. It is composed of a mineral dihydrate with the chemical formula CaSO4 2H2O.

It is widely mined for its manufacture and used in the preparation of fertilizers and as a main ingredient in plaster, blackboard/pavement chalk, drywall, etc. In many places such as Egypt, Mesopotamia, Rome, and England, gypsum is mainly used to make sculptures and objects of other cultures. Its selenite is crystallized by a process called crystalline crystallization.

The positive molecule of water in the gypsum attracts the negative molecule of oxygen. This is because hydrogen bonding will be stronger only if the temperature remains cool.

Gypsum becomes unstable when heat destroys hydrogen bonds and removes water from hydration. Gypsum is a very soft mineral and is a form of calcium sulfate.

Gypsum is a Greek (gypsos) word derived from the word plaster. Because the Montmartre district, located in Paris, where the mine has been operating for a long time, burnt gypsum is used for many purposes.

From here, it came to be known as the Plaster of Paris. This gypsum is a dehydrated mineral.

Gypsum comes in its form only a few minutes after adding water to gypsum. The material from which it is made is used in the manufacture of castings.

gypsum

What is Anhydrite?

Anhydrite is a mineral that is the basic form of calcium sulfate and is an anhydrous mineral. Its chemical formula is CaSO4.

It is in the shape of an orthorhombic crystal with three complete cleavage directions. It has a Mohs hardness of 3.5 and a gravity measurement of 2.9.

Its colour varies, i.e. white, brown, blue, purple, etc. After developing the three cracks present in it, it shines like a pearl, and its second surface is glass.

This mineral completely turns into gypsum upon contact with water, and this change is reversible. Anhydrite is also associated mainly with other minerals such as calcite, halite and sulfide, galena, chalcopyrite, molybdenite, pyrite, etc.

It does not react with hydrochloric acid. It is a hard mineral. It is a hard crystal with a hardness density of 3.0.

It is considered a type of rare mineral. It is also known as hydrous calcium sulfate.

A. G. Warner gave this term in 1804. Earlier, some obsolete names were given to its species, such as Muriasite and Carstenite etc.

1-3 percent of the anhydrite is present in the salt domes; the remainder is left as a cap. The halite present in the water fades away.

anhydrite

Main Differences Between Gypsum and Anhydrite

  1. Gypsum is a mineral that is aqueous and contains 2 water molecules, while anhydrite, as it is known, is an anhydrous mineral and does not contain water molecules.
  2. The chemical formula of gypsum is (CaSO4 2H2O). On the other hand, the chemical formula of anhydrite is (CaSO4).
  3. Gypsum is softer, whereas anhydrite is a hard crystal.
  4. The hardness rating of gypsum is about 2, while If we look at its hardness rating of anhydrite, then it is 3.5. 
  5. The density of gypsum is about 2.33g/cc, whereas the density of anhydrite is 3.2g/cc.
  6. Gypsum has a monoclinic structural form, while anhydrite has an orthorhombic structural form. 
Difference Between Gypsum and Anhydrite
References
  1. https://scholarcommons.usf.edu/ijs/vol25/iss3/2/
  2. https://www.journals.uchicago.edu/doi/abs/10.1086/515996
  3. https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/34/3/512/95719

Last Updated : 11 June, 2023

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