PORTLANDITE

    Class : Oxides and hydroxides
    Subclass : Hydroxides and hydrates
    Crystal system : Trigonal
    Chemistry : Ca(OH)2
    Rarity : Rare to uncommon

Portlandite is a calcium hydrate which forms in very diverse environments, having in common strong temperature variations : contact metamorphism, volcanic fumaroles, alkaline hot springs, combustion zones of coal or bitumen seams. It is also found in industrial slag. Its name comes from Portland cement, whose solidification by hydration is due to massive crystallization of portlandite. It is a mineral which forms hexagonal lamellae of up to 6 cm, more commonly fibrous and porous masses.

Main photo : Portlandite from Wessels Mine, South Africa © Jeff Weissman

Portlandite in the World

The most beautiful samples of portlandite come from metamorphic manganese deposits in the Hotazel region (South Africa): these are crystals measuring up to 56 mm and imposing masses. Beautiful crystals are also known in Israel from the Hatrurim formation. Portlandite is above all a mineral produced artificially in very large quantities in cement factories around the world (more than a billion tonnes per year); the "setting" of the concrete results in fact from the formation by hydration of innumerable portlandite crystals whose entanglement with the grains of sand and gravel ensures the cohesion and rigidity of the whole.

Portlandite in France

In France, portlandite is known at Boisséjour near Clermont-Ferrand (Puy-de-Dôme) as well as in the slags of Crozet (Loire) and Lapanouse (Aveyron).

Right photo : Portlandite of Lapanouse slags, Aveyron, France @ Daniel & Pascale Journet

Twinning

No twins known for this mineral species.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 2 to 3
Density : 2.22 to 2.23
Fracture : Undetermined
Streak : White

TP : Transparent
RI : 1.547 to 1.575
Birefringence : 0.028
Optical character : Uniaxial -
Pleochroism : None
Fluorescence : None

Solubility : Water, hydrochloric acid

Magnetism : NoneRadioactivity : None

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