portlandite

{{Infobox mineral | name = Portlandite | category = Oxide mineral | image = Portlandite, Ettringite - Mineralogisches Museum Bonn.jpg | imagesize = | alt = | caption = Portlandite and ettringite | formula = Ca(OH)2 | IMAsymbol = Por | molweight = | strunz = 4.FE.05 | dana = 06.02.01.04 Brucite group | system = Trigonal | class = Hexagonal scalenohedral (m) H-M symbol: ( 2/m) | symmetry = Pm1 | unit cell = a = 3.589 Å, c = 4.911 Å; Z = 1 | color = Colorless, white to greenish white | colour = | habit = Hexagonal plates; commonly fibrous, powdery, massive. | twinning = | cleavage = Perfect on {0001} | fracture = | tenacity = Sectile with flexible cleavage plates | mohs = 2 | luster = Pearly on cleavages | streak = | diaphaneity = Transparent | gravity = 2.23 | density = | polish = | opticalprop = Uniaxial (−) | refractive = nω = 1.574 nε = 1.547 | birefringence = δ = 0.027 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = Soluble in water producing an alkaline solution | other = | alteration = Alters to CaCO3 on exposure to CO2 bearing waters | references = }} 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).

==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. 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.