• Chemistry: Ca6Al2(SO4)3(OH)12 - 26H2O, Hydrated Calcium Aluminum Sulfate Hydroxide.
  • Class: Sulfates
  • Group: Ettringite
  • Uses: Only as mineral specimens.
  • Specimens

Ettringite is a beautiful and interesting mineral for collectors. It forms as a precipitate from hydrothermal solutions. It has a typically bright yellow color and a nice luster. For collectors who like good crystal formations, ettringite has that too. It will commonly form well shaped hexagonal prisms that are topped by a hexagonal pyramid, which in turn is often truncated by the flat face of a pinacoid.

Its chemistry is interesting. Four out of every five atoms in this mineral is either a part of a water molecule or an hydroxide. It's almost all water! This fact is reflected in its very low specific gravity of only 1.7, that's less than twice the specific gravity of water.

It is difficult under ordinary means to distinguish ettringite from its close cousin sturmanite. Both are members of the Ettringite Group and have similar crystal habit, density, luster and will often share the same bright yellow color.


  • Color is a bright yellow, also colorless and white.
  • Luster is vitreous.
  • Transparency: Crystals are transparent to translucent.
  • Crystal System is trigonal; bar 3 2/m.
  • Crystal Habits include hexagonal prisms terminated by an hexagonal pyramid or a pinacoid, more commonly by both. Terminations can be rounded or dome-like but many have nice flat faces. Flattened rhombohedrons, acicular and fibrous forms are also seen.
  • Cleavage is poor and rarely seen.
  • Fracture is uneven.
  • Hardness is 2 - 2.5
  • Specific Gravity is approximately 1.7+ (well below average)
  • Streak is white.
  • Other Characteristics: Crystals are usually rather small, typically less than 1/4 inch long.
  • Associated Minerals are calcite, afwillite, pillipsite and some other zeolites.
  • Notable Occurrences include several South African localities such as Hotazel and Kuruman District and Ettringen, Germany (hence the name).
  • Best Field Indicators are crystal habit, density, hardness, lack of cleavage and color.
ETTRINGITE specimens:
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ETTRINGITE specimen ett-1
$ 110.00
Dims: 1.9" x 1.2" x 0.9" (4.8 x 3.0 x 2.3 cm)
Wt: 1.52 oz. (43.2 g)
N'Chwaning Mine, Hotazel, Kuruman, South Africa
This specimen contains over 10 crystals of Ettringite, a rather unusual sulfate mineral from South Africa. They are in generally good condition, though some of the crystasls show obvious damage, and some of that is severe. All have a hexagonal prismatic form with shallow pyramidal terminations, and most are double terminated. Their color is bright yellow and they have a vitreous luster. All seem to have a thin shell of transparent material that surrounds a translucent core. They rest on a host rock that appears to be made of a dark metallic oxide, as it is rather heavy for its size, and the Ettringite crystals are known to have a very low specific gravity. Many tiny, transparent quartz crystals also rest on this host rock.
no photo
ett-1 ($110.00)
N'Chwaning Mine, Hotazel, Kuruman, South Africa
ETTRINGITE specimen ett-2
$ 280.00
Dims: 1.2 x 1.1 x 1.0" (3.0 x 2.8 x 2.5 cm)
Wt: 1.06 oz. (30.2 g) w/ specimen box
Kalahari Fld., South Africa
Over 30 hexagonal prismatic Ettringite crystals rest on the dark brown host rock of this thumbnail specimen. These crystals range in size from a few millimeters along each axis to 0.4 x 0.3 x 0.3" (1.0 x 0.8 x 0.7 cm). Though a few are obviously broken and mostly incomplete, those that are intact are in excellent condition and have excellent form, with well-defined edges and clean faces. They have the classic bright yellow color and pearly luster of their species, and the material just beneath their surfaces is dimly transparent, surrounding a translucent "core". They rest on a dark brown host rock that I cannot identify, which is hot-glued onto a foam base. This foam base fits into the bottom of a standard cubic plastic specimen box.
no photo
ett-2 ($280.00)
Kalahari Fld., South Africa
ETTRINGITE specimen ett-3
$ 120.00
Dims: 1.7 x 1.3 x 1.2" (4.3 x 3.3 x 3.0 cm)
Wt: 1.0 oz. (29 g)
N'Chwaning Mine, Cape Province, South Africa
This thumbnail piece is made up mostly of Ettringite by weight. Though most of the exposed material is heavily damaged and incomplete, there are several small crystals that are in excellent condition. These do not tend to exceed 0.3" (8 mm) in length or diameter. All show excellent hexagonal prismatic form and the classic lemon-yellow color and pearly-to-vitreous luster. They are also generally transparent, though internal fractures dim and spoil their clarity. A rust-red material accompanies the Ettringite- it shows some micro-crystalline form, but I cannot identify it.
no photo
ett-3 ($120.00)
N'Chwaning Mine, Cape Province, South Africa
ETTRINGITE specimen ett-4
$ 50.00
Dims: 5.6x1.2x0.9" (14.2x3.0x2.2 cm)
Wt: 3.9 oz. (109.6g)
N'Chwaning Mine, South Africa
This is a really interesting specimen. Its core is filled with thin black blades of some mineral with a vitreous luster (looks a bit like schorl). These crystals also form a dusting on the outer surface. The bulk of the specimen is a radiating mass of crystals (pale brown, opaque, a soft woody feel) which looks like a wooden stick, but I believe is a mineral of some type. Over this is a crust of small, light brown bruceite crystals. These are translucent to dimly transparent and have a pearly luster. And on this cylinder (perhaps it was a stalactite) are several crystals of ettringite. While most of the ettringite is colorless or white, the outer layer (where it still remains) is a vibrant yellow. There is a scattering of very small bright yellow ettringite crystals scattered over the specimen. This specimen has been repaired - the stalactite appears to have broken into 4 pieces which were glued back together.
no photo
ett-4 ($ 50.00)
N'Chwaning Mine, South Africa
ETTRINGITE specimen ett-5
$ 60.00
Dims: 2.21x1.41x1.02" (5.62x3.59x2.60cm)
Wt: 1.17oz (33.2g)
N'Chwaning Mine, South Africa
A cluster of clear quartz crystals is host to numerous intergrown crystals of ettringite. These crystals are hexagonal with complex faceted dome terminations, they are translucent, and yellow in color. The largest of these is about 7mm in diameter, and most of the crystals are 1 or 2mm long.
no photo
ett-5 ($ 60.00)
N'Chwaning Mine, South Africa


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