Anhydrite

This is a white rock.

It can look grey or even blue. Some pieces look like glass.

This rock has no water inside it. If it touches water, it changes. It turns into a rock called gypsum.

You can find it near salt. It often sits on top of salt domes. Some parts look like chicken wire.

It is a very interesting mineral to find.

Anhydrite is a white mineral.

It can also look grey, blue, or purple. Sometimes it looks like glass. Other times it has a pearly shine.

This mineral has no water inside it. This is why it is called anhydrite. The name means it lacks water. It is different from a mineral called gypsum. Gypsum does have water inside it.

Anhydrite and gypsum are linked by a special way. If anhydrite touches water, it absorbs it. Then it turns into gypsum. This can happen deep underground. You can also turn gypsum back into anhydrite. You must heat it up to do this.

Anhydrite is often found near salt. It can form a cap on top of salt domes. In some places, it looks like chicken wire. This happens when it replaces gypsum in the ground. Scientists also find it in some volcanic rocks. It was first found in a salt mine in 1794.

Some people use it for art. One artist made a carving from it.

Anhydrite is a special mineral that is often white in color. It can also look grey, blue, or even purple.

This mineral has a hardness of 3.5 on the Mohs scale. It has a specific gravity of 2.9. You might see it as small crystals. Most of the time, it appears as larger masses. Some surfaces have a glassy look. Other surfaces have a pearly shine. It is a type of calcium sulfate mineral.

This mineral works in a very interesting way with water. Anhydrite does not have water inside it. If it touches water, it absorbs it. This causes it to transform into gypsum. This change can also work in reverse. If you heat gypsum, it can become anhydrite again.

In the ground, depth is very important for this. Near the surface, water often turns anhydrite into gypsum. In salt basins, it forms in special ways. It can even form in tidal flats.

People have studied this mineral for a long time. A man named A. G. Werner named it in 1804. He chose the name because it lacks water.

Before that, people used other names for it. One old name was muriacite. Another name used in the past was karstenite. The mineral was first discovered in 1794. This happened in a salt mine near Hall in Tirol. It is often found near other minerals like calcite.

Anhydrite appears in many different places on Earth. It is often found in salt domes. It can make up 1% to 3% of a salt dome. It often stays as a cap at the top of the salt.

You can find it in volcanic rocks too. It was found in El Chichón volcano in Mexico. It was also found in El Teniente in Chile. In some areas, it looks like chicken wire. This happens when it replaces gypsum in the ground. These shapes are called nodules.

This mineral connects to many things we see every day. Some people use it for art and carvings. One artist made a relief carving of a kiln. There is also a blue-grey variety called angelite.

Some parts of the mineral are used for decoration. A variety called vulpinite is polished for ornaments. In the past, it was even used in buildings. Understanding how it changes helps us understand the ground. It shows how water and heat change our world.

Anhydrite is a mineral composed of anhydrous calcium sulfate. Its chemical formula is CaSO4. This means it is a calcium sulfate without water. It belongs to the orthorhombic crystal system. This system describes how its internal atoms are arranged. The mineral has three directions of perfect cleavage. This means it breaks easily along three specific flat planes. It is not isomorphous with baryte or celestine. Even though those minerals have similar formulas, their structures are different.

Anhydrite undergoes a unique chemical transformation when it meets water. It readily absorbs water to become gypsum. Gypsum has the formula CaSO4·2H2O. This means it contains two molecules of water. This process is reversible. You can turn gypsum back into anhydrite by heating it. This happens under normal atmospheric conditions. In nature, anhydrite forms from aqueous solutions. If the solution has too much sodium or potassium chloride, anhydrite forms at temperatures above 40°C. This is how the mineral forms naturally in salt basins.

Geologists find anhydrite in several distinct environments. It is often found in evaporite deposits alongside gypsum. One famous location is a salt mine near Hall in Tirol. In these deposits, depth is a very important factor. Near the surface, groundwater often turns anhydrite into gypsum. Anhydrite also forms in tidal flat environments. In the Persian Gulf sabkhas, it forms as massive replacement nodules. These nodules often have a netted appearance. Scientists call this specific texture "chicken-wire anhydrite."

Another common setting is within salt domes. Anhydrite makes up about 1% to 3% of the minerals in these domes. It often remains as a caprock at the top of the salt. This happens when pore waters remove the halite. A typical caprock has a specific order of layers. It starts with salt at the bottom. Above that is a layer of anhydrite. Then comes a layer of gypsum patches. Finally, a layer of calcite sits on top.

History shows us how our understanding of this mineral has changed. A man named A. G. Werner named it in 1804. He chose "anhydrite" because it lacks the water of crystallization found in gypsum. In the past, people used different names. One obsolete name was muriacite. This name was used because people thought it was a chloride. Another old name was karstenite. There are also specific names for different varieties of the mineral. A contorted variety is called tripe-stone. A scaly variety from Italy is called vulpinite.

Anhydrite has physical properties that help scientists identify it. It has a Mohs hardness of 3.5. This means it is relatively soft. Its specific gravity is 2.9. The color is usually white. However, it can also appear greyish, bluish, or purple. The luster changes depending on the surface. On its best cleavage planes, the luster is pearly. On other surfaces, it looks glassy. A semi-transparent light blue-grey variety from Peru is sold as angelite. This variety is quite beautiful.

Understanding anhydrite is important for studying Earth's systems. In oil fields, it can interact with hydrocarbons. At high temperatures, this interaction can cause thermochemical sulfate reduction, or TSR. This process reduces sulfate into hydrogen sulfide. It also causes calcite to precipitate. Anhydrite can even be found in igneous rocks. It was found in the El Teniente pluton in Chile. It was also found in the trachyandesite pumice of the El Chichón volcano in Mexico. Even the movement of water can cause problems. In 2007, drilling in Germany allowed water to hit an anhydrite layer. This caused the ground to swell as the mineral turned into gypsum.