Portlandite

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Portlandite
Portlandite, Ettringite - Mineralogisches Museum Bonn.jpg
Portlandite and ettringite
General
Category Oxide mineral
Formula
(repeating unit)
Ca(OH)2
Strunz classification 04.FE.05
Dana classification 06.02.01.04 Brucite group
Crystal symmetry Trigonal (3 2/m) - hexagonal scalenohedral
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.
Crystal system Trigonal
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 [1][2][3][4]

Portlandite is a rare oxide mineral, the naturally occurring form of calcium hydroxide (Ca(OH)2). It is the calcium analogue of brucite (Mg(OH)2).

Occurrence[edit]

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 larnitespurrite. 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.[1] 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.[2][1]

It occurs in association with afwillite, calcite, larnite, spurrite, halite, brownmillerite, hydrocalumite, mayenite and ettringite.[1]

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 hydrolysis product of Portland cement.[2][1]

References[edit]

  1. ^ a b c d e Handbook of Mineralogy
  2. ^ a b c Portlandite at Mindat.org
  3. ^ Portlandite at Webmineral
  4. ^ Palache, C., H. Berman, and C. Frondel, Dana’s System of Mineralogy, Wiley, 1944, (7th edition), v. I, pp. 641–642