Afwillite

Afwillite is a calcium hydroxide nesosilicate mineral with formula Ca₃Si₂O₄(OH)₆. It occurs as glassy, colorless to white prismatic monoclinic crystals. Its Mohs scale rating is 4.5. It occurs as an alteration mineral in contact metamorphism of limestone.

It was first described in 1925 for an occurrence in the Dutoitspan Mine, Kimberley, South Africa and was named for Alpheus Fuller Williams (1874-1953), a past official of the De Beers diamond company.

External links

• Mindat with location data
• Webmineral data
• American Mineralogist PDF

Abstract
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. Introduction
Afwillite’s major economic importance is in the production of cement. Over 95% of all hydraulic cement that is used in construction is Portland cement (Environmental Protection Agency 1995). Gray Portland cement is the most common type of Portland cement produced and its primary use is in construction (Environmental Protection Agency 1995). Portland cement is used in the construction of houses, roads and buildings, and because of this Portland cement has a major impact on the economy. Portland cements are very high in silica and calcium, and afwillite serves as an excellent source for providing these initial ingredients.

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).

Spurrite Crystal Structure
Afwillite, as mentioned earlier, is generally formed in association with spurrite. Spurrite also has some interesting characteristics. It is generally formed in contact zones from hot mafic magmas mixing with carbonate rocks (Smith 1960). Spurrite has a monoclinic structure, like afwillite, and its space group is P 2/a. Spurrite’s chemical formula is Ca5(SiO4)2(CO3) and it is biaxial with a birefringence of 0.0390-0.0400, giving second order red interference colors when viewed under crossed polarizers in a petrographic microscope (Barthemy 2000).

File:Spurrite.GIF
Figure 2 Crystal structure of Spurrite (Smith 1960)
The calcium is in six-fold coordination with the oxygen, the silicon is in a four-fold coordination with the oxygen and the carbon is in two-fold coordination. Figure 2 illustrates how this would look. The diagram properly demonstrates the Ca’s 6-fold coordination with respect to oxygen and also visually interrupts the Si coordination as well as the carbons coordination in regards the oxygen. One unique characteristic of spurrite is that it actually abides by two twin laws. Polysynthetic twinning can occur along its (001) and another type of twinning can occur parallel to its optical axes (Smith 1960). Spurrite is also commonly used in Portland cements because it is also enriched in calcium and silica, although afwillite is more commonly used. 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).

Alpheus Williams Fuller Biography
Alpheus Williams Fuller wrote two books on the formation of diamonds and developed three theories on their formation. In his book, Williams discusses these theories and aligns himself with the phenocryst school of thought. Alpheus gives a substantial analysis as to how diamonds are formed and how they vary in production. He believed that diamonds were formed from kimberlite (Moore 2004). It was probably because of his work with diamonds that afwillite was named after him. Alpheus had an excellent track record while in charge of the mines during the war, and it was because of this that he got the job as a mine manager for DeBeers. He attended Cornell University, in Ithaca, New York. (Cornell 1902).

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
Cornell (1902) Cornell Alumni News. 20, 3
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
Moore, T. (2004) A Collection of DIAMOND CRYSTALS with Notes on the Science, History, and Worldwide Localities of Diamonds. Miner. Rec. Pg 7
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