Mineral Information and Data:

Ettringite
Ca6Al2(SO4)3(OH)12 • 26H2O [CNMNC approved formula]

Named for the occurrence near Ettringen, Germany. First published in 1874. Species approved (IMA, 1960).

IMA status : Approved Species 1960 ()
CLASSIFICATION 
Dana
(8th edition) :
31.10.02.01  
  (31) Hydrated Sulfates Containing Hydroxyl or Halogen
  (31.10) miscellaneous
  (31.10.02)
  
Nickel-Strunz
(10th edition) :
07.DG.15
  (07) Sulfates (selenates, tellurates, chromates, molybdates, wolframates)
  (07.D) Sulfates (selenates, etc.) with additional anions, with H2O
  (07.DG) With large and medium-sized cations; with NO3, CO3, B(OH)4, SiO4 or IO3
  
Crystal system: Trigonal System
Point group (H-M): 3m — ditrigonal-pyramidal
Space group: P31c
Unit cell: a = 11.23Å, c = 21.44Å, Z=2
Crystal Habit: Crystals prismatic and striated along [0001], to 20 cm, commonly unterminated, rarely dipyramidal {1012}; fibrous to cottonlike.
  
Color: Colorless, pale yellow, milky white; colorless in transmitted light
Diaphaneity: Transparent to opaque
Luster: Vitreous
Hardness (Mohs): 2 - 2.5
Measured Density: 1.77 g/cm3
Cleavage: On {1010}, perfect
  
Geologic Setting: In metamorphosed limestone near igneous contacts or in xenoliths; as weathering crusts on larnite rocks (Hatrurim Formation, Israel).
Mineral Association: Portlandite, afwillite, hydrocalumite (Scawt Hill, Ireland); afwillite, hydrocalumite, mayenite, gypsum (Hatrurim Formation, Israel)
  
Other Properties: H2O Soluble. Gemstone Extremely Rare.
Comments: Partially soluble in H2O; partially dehydrates on exposure, becoming opaque.

 

Ettringite
Origin: Wessels Mine (Wessel's Mine), Hotazel, Kalahari manganese fields, Northern Cape Province, South Africa
Sample size: 24 x 16 x 13 mm
Photo: Dan Weinrich

 

Ettringite
(zoom-in of above)

 

Ettringite
Origin: Wessels Mine, near Kuruman, South Africa
Sample size: 8.5 x 4.6 x 1.5 cm
Photo: Rob Lavinsky

 

Ettringite
(zoom-in of above)

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