Gypsum and anhydrite minerals are quite well known, especially among collectors, making them a very popular mineral. Both are forms of calcium sulfate in a way. There are many differences between the two, which are explained in detail below. Both these minerals have been used as examples in textbooks and their characteristics have been highlighted.
Gypsum vs Anhydrite
The main 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).
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 Between Gypsum and Anhydrite
| Parameters of comparison | Gypsum | Anhydrite |
| Definition | Gypsum is a mineral that is aqueous and contains 2 water molecules. | Anhydrite, as it is known, is a mineral that is anhydrous 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 form | It 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, and drywall, etc. In many places such as Egypt, Mesopotamia, Rome, England, gypsum is mainly used as an ingredient in making 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 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.
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 color 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, and 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. This term is given by A. G. Warner 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 and the remainder is left as a cap. The halite present in the water fades away.
Main Differences Between Gypsum and Anhydrite
- 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 is softer whereas anhydrite is a hard crystal.
- The hardness rating of gypsum is about 2 while If we look at its hardness rating of anhydrite, then it is 3.5.
- The density of gypsum is about 2.33g/cc whereas the density of anhydrite is 3.2g/cc.
- Gypsum has a monoclinic structural form while anhydrite has an orthorhombic structural form.
Conclusion
So here we conclude that both the minerals are far different from each other. Both are forms of calcium sulfate in a way. 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. Gypsum has a monoclinic structural form while anhydrite has an orthorhombic structural form. The chemical formula is also different. The chemical formula of gypsum is (CaSO4 2H2O). On the other hand, the chemical formula of anhydrite is (CaSO4). The hardness rating of gypsum is about 2 while If we look at its hardness rating of anhydrite, then it is 3.5.
References
- https://scholarcommons.usf.edu/ijs/vol25/iss3/2/
- https://www.journals.uchicago.edu/doi/abs/10.1086/515996
- https://pubs.geoscienceworld.org/sepm/jsedres/article-abstract/34/3/512/95719