Portlandite

minerals

brownmillerite

mayenite

afwillite

Formula:Ca(OH)2
Hydroxide, brucite group
Specific gravity: 2.227 to 2.233
Hardness: 2½ to 3
Streak: White
Colour: Colourless, light yellow
Solubility: Slowly soluble in water, giving an alkaline solution, and completely soluble in weak hydrochloric acid Dana (AM 19.35)
Environments

Metamorphic environments
Fumeroles

Portlandite occurs in larnite-spurrite contact metamorphic rocks and volcanic fumaroles (Webmin). It is rare in nature, but an important phase in cement materials and concrete; it is indicative of a very alkaline environment, with a pH in excess of 11 (Mindat, AM 48.924-930).

Localities

At the Eifel district, Germany, portlandite is associated with calcite, brownmillerite, hydrocalumite, mayenite and ettringite (HOM).

In the Hatrurim region, Israel, portlandite occurs in sedimentary deposits metamorphosed at high-temperature and low-pressure by spontaneous combustion of bitumen (HOM), associated with fluorapatite and ettringite (Dana).

At Vesuvius, Italy, portlandite occurs in fumeroles (Dana, HOM).

At Jebel Awq, Oman, portlandite occurs as a precipitate from alkaline springs derived from ultramafic rocks, and associated with calcite and halite (HOM).

At the Chelyabinsk coal basin, Russia, portlandite occurs in burning coal measures (HOM).

At the type locality, Scawt Hill, Larne, County Antrim, Ulster, Northern Ireland, UK, portlandite is an alteration product of calcium silicates in larnite-spurrite contact metamorphic rocks, associated with afwillite, calcite, larnite and spurrite (HOM, Dana, AM 19.36). It is formed by the hydration of the calcium orthosilicates (AM 19.36).

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