Portlandite

Portlandite is a hydroxide-bearing mineral typically included in the oxide mineral class. It is the naturally occurring form of calcium hydroxide (Ca(OH)₂) and the calcium analogue of brucite (Mg(OH)₂).

Portlandite
Portlandite and ettringite
General
Category	Oxide mineral
Formula	Ca(OH)2
IMA symbol	Por[1]
Strunz classification	4.FE.05
Dana classification	06.02.01.04 Brucite group
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	P3m1
Unit cell	a = 3.589 Å, c = 4.911 Å; Z = 1
Identification
Color	Colorless, white to greenish white
Crystal habit	Hexagonal plates; commonly fibrous, powdery, massive.
Cleavage	Perfect on {0001}
Tenacity	Sectile with flexible cleavage plates
Mohs scale hardness	2
Luster	Pearly on cleavages
Diaphaneity	Transparent
Specific gravity	2.23
Optical properties	Uniaxial (−)
Refractive index	nω = 1.574 nε = 1.547
Birefringence	δ = 0.027
Solubility	Soluble in water producing an alkaline solution
Alters to	Alters to CaCO3 on exposure to CO2 bearing waters
References	[2][3][4][5][6]

Occurrence

Portlandite occurs in a variety of environments. At the type location in Northern Ireland, it occurs as an alteration of calc–silicate rocks by contact metamorphism of larnite–spurrite. It occurs as fumarole deposits in the Vesuvius area. In Jebel Awq, Oman, it occurs as precipitates from an alkaline spring emanating from ultramafic bedrock. In the Chelyabinsk coal basin of Russia it is produced by combustion of coal seams and similarly by spontaneous combustion of bitumen in the Hatrurim Formation of the Negev desert in Israel and the Maqarin area, Jordan.^[3] It also occurs in the manganese mining area of Kuruman, Cape Province, South Africa in the Kalahari Desert where it occurs as large crystals and masses.^[4][3]

It occurs in association with afwillite, calcite, larnite, spurrite, halite, brownmillerite, hydrocalumite, mayenite and ettringite.^[3]

It was first described in 1933 for an occurrence at Scawt Hill, Larne, County Antrim, Northern Ireland. It was named portlandite because the chemical calcium hydroxide is a common hydration phase of Portland cement.^[4][3]