Monohydrocalcite is a mineral that is a hydrous form of calcium carbonate

Calcium carbonate

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

, CaCO₃·H₂O. It was formerly also known by the name hydrocalcite, which is now discredited by the IMA

International Mineralogical Association

The International Mineralogical Association is an international group of 38 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 4000 plus known mineral species...

. It is a trigonal

Rhombohedral crystal system

In crystallography, the trigonal crystal system is one of the seven crystal systems, and the rhombohedral lattice system is one of the seven lattice systems...

 mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

 which is white when pure. Monohydrocalcite is not a common rock-forming mineral, but is
frequently associated with other calcium and magnesium carbonate minerals

Carbonate minerals

Carbonate minerals are those minerals containing the carbonate ion: CO32-.-Anhydrous carbonates:*Calcite group: Trigonal**Calcite CaCO3**Gaspeite CO3**Magnesite MgCO3**Otavite CdCO3**Rhodochrosite MnCO3**Siderite FeCO3**Smithsonite ZnCO3...

, such as calcite

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

, aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

, lansfordite

Lansfordite

Lansfordite is a hydrated magnesium carbonate mineral with composition: MgCO3·5H2O. Landsfordite was discovered in 1888 in a coal mine in Lansford, Pennsylvania. It crystallizes in the monoclinic system and typically occurs as colorless to white prismatic crystals and stalactitic masses. It is a...

, and nesquehonite.

Monohydrocalcite has been observed in air conditioning systems, and in moonmilk

Moonmilk

Moonmilk is a white, creamy substance found inside caves. It is similar to other deposits, but its unique quality is that it does not harden or turn to stone...

 deposits in caves, both probably formed from spray of carbonate rich fluids.
It is well-known in Robe

Robe, South Australia

Robe is a town and fishing port on the South East Limestone Coast of South Australia. The town's distinctive combination of historical buildings, ocean, fishing fleets, lakes and dense bush is widely appreciated. Robe lies on the southern shore of Guichen Bay, just off the Princes Highway. At the...

 on the Limestone Coast

Limestone Coast

The Limestone Coast is a name used since the early twentyfirst century for the geographical region and tourist region of the southeast coast of South Australia from the Victoria border to the towards the city of Adelaide.-Location and description:...

 of South Australia

South Australia

South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...

 as a component of beach sands of Lake Fellmongery and Lake Butler,
where it is believed to be formed from algal spume.