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Formula:Ca(OH)2
Hydroxide, brucite group
Crystal System: Trigonal
Specific gravity: 2.227 to 2.233 measured, 2.26 calculated
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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