Portlandite is a rare oxide mineral

Oxide mineral

The oxide mineral class includes those minerals in which the oxide anion is bonded to one or more metal ions. The hydroxide bearing minerals are typically included in the oxide class...

, the naturally occurring form of calcium hydroxide

Calcium hydroxide

Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...

 (Ca(OH)₂). It is the calcium analogue of brucite

Brucite

Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and formed during serpentinization of dunites...

 (Mg(OH)₂).

Occurrence

Portlandite occurs in a variety of environments. At the type location

Type locality (geology)

Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....

 in Northern Ireland it occurs as an alteration of calc-silicate rocks by contact metamorphism of larnite

Larnite

Larnite is a mineral, known as dicalcisum silicate.Dicalcium silicate is chemically, β–Ca2SiO4, sometimes represented by the formula 2CaO·SiO2.When used in the cement industry, the mineral is usually referred to as Belite....

–spurrite

Spurrite

Spurrite is a white, yellow or light blue mineral with monoclinic crystals. Its chemical formula is Ca52CO3.Spurrite is generally formed in contact metamorphism zones as mafic magmas are intruded into carbonate rocks. Spurrite's space group is P 2/a...

. It occurs a fumarole

Fumarole

A fumarole is an opening in a planet's crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from...

 deposits in the Vesuvius area. In Jebel Awq, Oman

Oman

Oman , officially called the Sultanate of Oman , is an Arab state in southwest Asia on the southeast coast of the Arabian Peninsula. It is bordered by the United Arab Emirates to the northwest, Saudi Arabia to the west, and Yemen to the southwest. The coast is formed by the Arabian Sea on the...

 it occurs as precipitates from an alkaline spring emanating from ultramafic bedrock. In the Chelyabinsk

Chelyabinsk

Chelyabinsk is a city and the administrative center of Chelyabinsk Oblast, Russia, located in the northwestern side of the oblast, south of Yekaterinburg, just to the east of the Ural Mountains, on the Miass River. Population: -History:...

 coal basin of Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

 it is produced by combustion of coal

Coal

Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

 seams and similarly by spontaneous combustion of bitumen in the Hatrurim Formation of the Negev desert in Israel

Israel

The State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...

 and the Maqarin area, Jordan

Jordan

Jordan , officially the Hashemite Kingdom of Jordan , Al-Mamlaka al-Urduniyya al-Hashemiyya) is a kingdom on the East Bank of the River Jordan. The country borders Saudi Arabia to the east and south-east, Iraq to the north-east, Syria to the north and the West Bank and Israel to the west, sharing...

. It also occurs in the Kalahari manganese

Manganese

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

 mining area of Kuruman

Kuruman

Kuruman is a town with 12,701 inhabitants in Northern Cape province of South Africa, famous for its scenic beauty and the Eye of Kuruman, a geological feature bringing water from deep underground to the surface in the Kalahari Desert....

, Cape Province

Cape Province

The Province of the Cape of Good Hope was a province in the Union of South Africa and subsequently the Republic of South Africa...

, South Africa

South Africa

The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

 where it occurs as large crystals and masses.

It occurs in association with afwillite

Afwillite

Afwillite is a calcium hydroxide nesosilicate mineral with formula Ca32.2H2O. It occurs as glassy, colorless to white prismatic monoclinic crystals. Its Mohs scale hardness is between 3 and 4...

, 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:...

, larnite, spurrite, halite

Halite

Halite , commonly known as rock salt, is the mineral form of sodium chloride . Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount and type of impurities...

,
brownmillerite

Brownmillerite

Brownmillerite is a mineral with an interesting structure. Its chemical formula is Ca22O5. It is named for Lorrin Thomas Brownmiller , chief chemist of the Alpha Portland Cement Company, Easton, Pennsylvania....

, hydrocalumite, mayenite

Mayenite

Mayenite , Ca12Al14O33, is a rare cubic mineral of cubic symmetry, originally reported from Eifel volcanic complex . It is also found at pyrometamorphic sites: e.g., in the Hatrurim Formation and in some burned coal dumps. The formula can be written as [Ca12Al14O32]O, which refers to the unique...

 and ettringite

Ettringite

Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2312·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system...

.

It was first described in 1933 for an occurrence at Scawt Hill, Larne

Larne

Larne is a substantial seaport and industrial market town on the east coast of County Antrim, Northern Ireland with a population of 18,228 people in the 2001 Census. As of 2011, there are about 31,000 residents in the greater Larne area. It has been used as a seaport for over 1,000 years, and is...

, County Antrim

County Antrim

County Antrim is one of six counties that form Northern Ireland, situated in the north-east of the island of Ireland. Adjoined to the north-east shore of Lough Neagh, the county covers an area of 2,844 km², with a population of approximately 616,000...

, Northern Ireland

Northern Ireland

Northern Ireland is one of the four countries of the United Kingdom. Situated in the north-east of the island of Ireland, it shares a border with the Republic of Ireland to the south and west...

. It was named portlandite because the chemical calcium hydroxide is a common hydrolysis product of Portland cement

Portland cement

Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...

.