Ettringite

Ettringite was first described in 1874 by J.Lehmann, for an occurrence near the Ettringer Bellerberg Volcano, Ettringen, Rheinland-Pfalz, Germany. It occurs within metamorphically altered limestone adjacent to igneous intrusive rocks or within xenoliths. It also occurs as weathering crusts on larnite in the Hatrurim Formation of Israel. It occurs associated with portlandite, afwillite and hydrocalumite at Scawt Hill, Ireland and with afwillite, hydrocalumite, mayenite and gypsum in the Hatrurim Formation. It has also been reported from the Zeilberg quarry, Maroldsweisach, Bavaria; at Boisséjour, near Clermont-Ferrand, Puy-de-Dôme, Auvergne, France; the N’Chwaning mine, Kuruman district, Cape Province, South Africa; in the US, occurrences were found in spurrite-merwinite-gehlenite skarn at the 910 level of the Commercial quarry, Crestmore, Riverside County, California and in the Lucky Cuss mine, Tombstone, Arizona.

The mineral ettringite has a structure that runs parallel to the c axis -the needle axis-; in the middle of these two lie the sulfate ions and H2O molecules, the space group is P31c. Ettringite crystal system is trigonal, crystals are elongated and in a needle like shape, occurrence of disorder or twining is common, which affects the intercolumn material. The first X-ray study was done by Bannister, Hey & Bernal (1936), which found that the crystal unit cell is of a hexagonal form with a=11.26 and c=21.48 with space group P63/mmcand Z=2. From observations on dehydration and chemical formulas there were suggestions of the structure being composed of Ca and Al(OH)6, were between them lie SO4 ions and H2O molecules. Further X-ray studies ensued; namely Wellin (1956) which determined the crystal structure of thaumasite, and Besjak & Jelenic (1966) which gave affirmation of the structure nature of ettringite. An ettringite sample extracted from Scawt Hill was analysed by C. E. Tilley, the crystal was 1.1 x 0.8 0.5 mm, with specific gravity of 1.772±0.002, the density was determined by immersing the sample in a solution of carbon tetrachloride mixed with bromoform. The crystal showed five prism faces of the form m {1010} and a small face a{1120}, with no pyramidal or basal faces, upon x-ray diffraction a Lauer gram along the c-axis revealed a hexagonal axis with vertical planes of symmetry, this study showed that the structure has a hexagonal and not a rhombohedral lattice. further studies conducted on synthetic ettringite by use of x-ray and powder diffraction confirmed earlier assumptions and analyses. Upon analyzing the structure of both ettringite and thaumasite, it was deduced that both minerals have hexagonal structures, but different space groups. Ettringite crystals have a P31c with a=11.224 Å, c= 21,108 Å, while thaumasite crystals fall into space group P63 with a= 11.04 Å, c= 10.39 Å. While these two minerals form a solid solution, the difference in space groups lead to discontinuities in unit cell parameters. Differences between structures of ettringite and thaumasite arise from the columns of cations and anions: Ettringite cation columns are composed of Ca3[Al(OH)6·12H2O], which run parallel to the c axis, and the other columns of sulfate anions and water molecules in channels parallel to these columns. In contrast, thaumasite consists of a cylindrical column of Ca3[Si(OH)6·12H2O] in the c axis, with sulfate and carbonate anions in channels between these columns which contain water molecules as well.

Ettringite occurs not only in nature, but is also a product of the cement-making process which helps give cement its rapid-setting properties. Research is currently underway to understand how this mineral may help remove heavy metals and dangerous toxic substances from soil.