Afwillite

Afwillite is important because it is used in Portland cement, a fundamental material used in construction. Additionally, afwillite has some unusual structural characteristics; hydrogen bonds join silica tetrahedra together in its crystal structure. It was named after Alpheus Fuller Williams. Afwillite is typically found in the veins of spurrite and it belongs to the nesosilicates sub-class. This subclass also includes the olivine group, the humite group, the zircon group and the garnet group. Afwillite’s chemical formula is Ca3Si2O4(OH)6 , so the mineral consists of about 49.13% CaO, 35.09% SiO2 and 15.78% H2O (weight percent oxides). It is in the monoclinic system, its space group is P2 and its point group is 2.

Formation of Afwillite

It is suggested that afwillite actually forms in fractured veins of the mineral spurrite. Jennite, afwillite, oyelite and calcite are all minerals that form in layers within spurrite veins. It appears that the afwillite, as well as the calcite, form from precipitated fluids. The jennite is actually an alteration of the afwillite; however, both formed from calcium silicates through hydration. From various tests, it has been determined that afwillite forms at a temperature below 200°C, usually roughly around 100°C (KUSACHI and HENMI 1989). Afwillite, as well as spurite, are formed through contact metamorphism of limestone (Barthemy 2000). Contact metamorphism is caused by the interaction of rock with heat and/or fluids from a nearby crystallizing silicate magma (Klein and Dutrow 2007).

Structure and properties

Afwillite has a very complex structure, it is monoclinic and the silicon tetrahedra in the crystal structure are actually held together by hydrogen bonds (Malik and Jeffery 1976 ). It has perfect cleavage parallel to its (101) and poor cleavage parallel to its (100) (Megaw 1976). It is biaxial and its 2V angle, the measurement from one optical axis to the other optical axis, measures 50 – 56 degrees. When viewed under crossed polarizers in a petrographic microscope, it displays first order orange colors, giving a maximum birefringence of 0.0167 (determined by using the Michel- Levy chart). Afwillite is optically positive (Web minerals). Additionally, it has a prismatic crystal habit (Henmi 1989). Under a microscope afwillite optically looks like wollastanite, which is in the same family as afwillite.

File:Afstructure.GIF

Figure 1 Crystal structure of afwillite (Megaw 1952)

Figure 1 shows the structure of afwillite. It also identifies the hydrogen bonds. Awfillite is composed of double chains that consist of calcium and silicon polyhedral connected to each other by sharing corners and edges. This causes continuous sheets to form parallel to its miller index [-101]. The sheets are bonded together by hydrogen bonds and are all connected by its Ca-Si-O bonds (Malik and Jeffery 1976). Each Ca is in 6-fold octahedral coordination with the O and the Si is in 4-fold tetrahedral coordination around the O. Around each Si there is one OH group and there are 3 Os that neighbor them (Malik and Jeffery 1976). The Si tetrahedra are arranged so that they share an edge with Ca(1) and Si(2) shares edges with the Ca(2) and Ca(3) polyhedral (Malik and Jeffery 1976.) The Si tetrahedra are held together by the OH group and hydrogen bonding occurs between the hydrogen in the OH and the Si tetrahedra. Hydrogen bonding is caused because of the positive charged ion, hydrogen is attractive to a negatively charge ion(s) which, in this case, are the Si tetrahedral (Klein and Dutrow 2007)

Uses of Afwillite

Portland cement is a gray or white powder and is composed hydrous materials such as afwillite. The cement gets its strength from the hydration of its Di- and tri- calcium silicates. The more potent the mix of these components is, the stronger the cement will be. Afwillite is a calcium silicate (Bye 1999), which makes it a very useful and desirable mineral for the manufacturing of Portland Cements. The grey Portland cement is generally used in construction because it has a higher concentration of iron and magnesium resulting in heavier, harder cement. White cement is typically used as a decorative because it has a lower iron and magnesium concentration (Environmental Protection Agency 1995). The cement is made by taking a mix of calcium carbonates along with aluminum silicates and burning them. It is then ground with gypsum and this produces the cement. When determining the quality of the cement, it is viewed using petrographic microscopy. The purpose of using petrography is to find defects in the cement. This is important because the crystal size can affect the cooling and hardening of the cement, and this can all be determined using optics (Bye 1999).

Work cited

The most highly cited source was Megaw’s article, The Structure of Awfillite. It was written in 1952, and is a follow up to his original publication on Afwillite. Most of the articles cited refer to Megaw’s article.

Barthemy, D. (2000) Afwillite Mineral Data, (http://webmineral.com/data/Afwillite.shtml)

Bye, C. G. (1999) Portland Cement. Composition, Production and Properties. 1.1.2, 2

Klein, C. and Dutrow, K. (2007) Manual of Mineral Science. 23rd Edition, 63, 596

Brown, D. (2000). Global whiteness. Pg 12

United States Environmental Protection Agency (1995). Portland AP 42, Fifth Edition

Compilation of Air Pollutant Emission Factors. 1, 11.6.1

Malik, K. M. A & Jeffery J. W. (1976) A Re-investigation of the Structure of Afwillite. Acta Cryst. B32, 475

Kusachi, I, Henmi, C. And Henmi K. (1989) Afwillite and jennite from Fuka, Okayama Prefecture. Japan. Miner J. 14, 279-292.

Megaw H. D. (1952). The structure of Afwilltie. Acta. 5, 477

Smith, J.V. (1960) The Crystal structure of Spurrite, Ca5(SiO4)2CO3. Acta. Cryst. 13, 454