Ettringite - Occurrence, Properties, and Distribution

Topics Covered

Introduction to Ettringite
Properties of Ettringite
How to Identify Ettringite
Global Distribution
Occurrence of Ettringite and Useful Mineral Association

Introduction to Ettringite

Chemical Formula – Ca6Al2(SO4)3(OH)12·26(H2O)

Ettringite is a hydrous calcium aluminum sulfate mineral first described in 1874 from Ettringer Bellerberg, Germany. The mineral was named after its place of discovery.

Properties of Ettringite

The following are the key properties of Ettringite:

  • Cell Data
    • Space Group: P31c
    • a = 11.23
    • c = 21.44
    • Z = 2
  • Crystal Data
    • Hexagonal
    • Point Group: 3m
    • Crystals prismatic and striated along [0001], to 20 cm, commonly unterminated, rarely dipyramidal {1012}; fibrous to cottonlike
    • X-ray Powder Pattern: 9.73 (100), 5.61 (80), 3.88 (50), 2.564 (45), 2.209 (45), 2.773 (40), 4.69 (35)
  • Chemical Composition
    Elements Content 1 Content 2
    H2O 46.3 45.93
    CaO 26.6 26.81
    SO3 18.8 19.14
    Al2O3 7.0 8.12
    CO2 0.8 -
    Total 99.5 100.00
  • Optical Properties
    • Optical Class: Uniaxial (–), changing to uniaxial (+) on dehydration
    • ω = 1.464
    • ε = 1.458
  • Estimated Properties
    Electron density Bulk density (electron density)=1.89 g/cm3
    note: Specific gravity of Ettringite =1.80 g/cm3
    Photoelectric PEEttringite = 2.97 barns/electron
    U= PEEttringite x ρElectron density= 5.60 barns/cm3
    Fermion index Fermion index = 0.001
    Boson index = 0.998
    Radioactivity Ettringite is not radioactive

How to Identify Ettringite

Ettringite is either colorless or white in color. It is transparent in appearance and non-magnetic. The mineral is non-fluorescent with white streak, perfect cleavage and vitreous luster. It forms needle-like crystals. The hardness of ettringite ranges from 2 to 2.5, and its density is 1.8 g/cm3.

Global Distribution

Ettringite is distributed in the following places:

  • Ettringer-Bellerberg volcano, near Mayen, and Schellkopf near Brenk, Eifel district, Germany
  • Zeilberg quarry, Maroldsweisach, Bavaria
  • Boisséjour, near Clermont-Ferrand, Puy-de-Dôme, France
  • Scawt Hill, near Larne, Co. Antrim, Ireland
  • Hatrurim Formation, Israel
  • N’Chwaning mine, Kuruman district, Cape Province, South Africa
  • Crestmore quarry, Riverside Co., California
  • Lucky Cuss mine, Tombstone, Cochise Co., Arizona

Occurrence of Ettringite and Useful Mineral Association

Ettringite occurs in xenoliths or metamorphosed limestone near igneous contacts. It also occurs as weathering crusts on larnite rocks.

The minerals that are closely associated with ettringite include gypsum, mayenite, hydrocalumite, afwillite, hydrocalumite and portlandite.


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