ETTRINGITE
Ettringite – Ca6Al2(SO4)3(OH)12·26H2O – is a very rare naturally occurring sulfate, but it is common as a synthetic phase in concrete. It is of no importance as an ore.
It is classified within the Ettringite Group. It is the aluminum analogue of bentonite and the sulfur analogue of siwaqaita.
Ettringite is readily soluble in dilute acids. It partially decomposes with water, forming an alkaline solution.
1. Characteristics
Crystal system: Trigonal, pyramidal ditrigonal.
Color: Colorless, yellow, greenish-yellow.
It turns white upon partial dehydration.
Habit: Small prismatic crystals; may lack terminal faces. Fibrous, resembling cotton.
Cleavage: {10-10} perfect. Striations along [0001].
Tenacity: Brittle.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 2 – 2.5
Parting: No.
Streak: Colorless.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 1.77
2. Geology and Deposits
At the type locality (Ettringer Feld, Germany), it occurs in cavities within metamorphosed limestone xenoliths hosted in leucite-nepheline tephrite. It also occurs in the contact zone between the tephrite and the limestone.
Ettringite also occurs in skarns containing spurrite, merwinite, and gehlenite (Crestmore, Canada).
It forms as an alteration product of larnite-bearing rocks (Hatrurim Formation, Israel).
Ettringite is a normal phase in concrete.
3. Mineral Associations
It occurs associated with quartz, calcite, barite, brucite, gypsum, fluorite, garnet (andradite), hematite and Mn minerals (hausmannite, manganite, rhodocrosite).
It also associates with some rarer minerals, such as oyelite, gaudefroyite, sturmanite, portlandite, afwilite, hydrocalumite, mayenite, spurrite, merwinite and gehlenite.
4. Transmitted Light Microscopy
Refraction indices: nω: 1.491 ou 1.464 nε: 1.470 ou 1.458
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless, without pleochroism.
Relief: Moderate.
Cleavage: {10-10} perfect.
Habits: Prismatic; it can be fibrous.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.021, corresponding to gray interference colors. White, yellow, and red, up to the blue at the end of the first order.
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: U(-), becomes U(+) when dehydrated.
2V angle: No.
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the recommended analytical method for identifying ettringite. However, preparing a polished section is important for identifying opaque minerals that may occur associated with ettringite, such as hematite, hausmannite, and manganite.
Sample preparation: xx
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: xx
General Characteristics: xx