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Ettringite
Current inventory:  0 gems
 

Chemistry:  Ca₆Al₂(SO₄)₃(OH)₁₂·26(H₂O)
[Hydrated Calcium Aluminum Sulfate Hydroxide]

Discovered in 1874;   IMA status: Valid (pre-IMA; Grandfathered).
Ettringite is named after the type locality at Ettringen, Rhineland-Palatinate, Germany.

Classification

Mineral Classification:

Sulfates

Strunz 8th Ed. ID:

6/D.13-10

Nickel-Strunz 10th Ed. ID:

7.DG.15

7 : SULFATES (selenates, tellurates, chromates, molybdates, wolframates)
D : Sulfates (selenates, etc.) with additional anions, with H₂O
G : With large and medium-sized cations; with NO₃, CO₃, B(OH)₄, SiO₄ or IO₃

Related to:

Ettringite Group

Crystal Data

Crystallography:

Hexagonal - Dihexagonal Dipyramidal

Crystal Habit:

Crystals prismatic and striated, to 20 cm, commonly unterminated, rarely dipyramidal; may be acicular, fibrous or cottonlike.

Twinning:

n/a

Physical Properties

Cleavage:

[1010] Perfect

Fracture:

Uneven

Tenacity:

Brittle

Hardness (Mohs):

2.0 - 2.5

Density:

1.77 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioacitve

Other:

Partially soluble in water; partially dehydrates on exposure, becoming opaque

Optical Properties

Color:

Yellow, pale Yellow, Colorless, milky White; turns White upon partial dehydration

Transparency:

Transparent to Opaque

Luster:

Vitreous

Refractive Index:

1.470 - 1.491  Uniaxial ( - ); changes to Uniaxial ( + ) upon dehydration

Birefringence:

0.0210

Dispersion:

n/a

Pleochroism:

n/a

Occurances

Geological Setting:

In cavities of metamorphosed limestone inclusions in a leucite-nepheline-tephrite rock.

Common Associations:

Afwillite, Gypsum, Hydrocalumite, Mayenite, Portlandite

Type Locality:

Ettringer Bellerberg Mt., Ettringen, Mayen, Eifel Mts, Rhineland-Palatinate, Germany

Year Discovered:

1824

View mineral photos:

Ettringite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Ettringite is an extremely rare gem and a member of the Ettingite Group of minerals that includes Charlesite, Ettringite, Sturmanite and Thaumasite. Ettringite crystals are usually very small so gems are very small. Ettringite is closely related to and difficult to distinguish from Sturmanite. Ettingite crystals are usually found as colorless but have been found as yellow to pale yellow from the source at the N’Chwaning mine, Kuruman district, Cape Province, South Africa. This location is currently the only source for gem quality crystals. Other locations of Ettringite only produce minute crystals too small for faceting.
 

  
Ettringite gems for sale:

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