Afwillite: Difference between revisions
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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. |
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. |
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Afwillite is typically found in the veins of [[spurrite]] and it belongs to the nesosilicate sub-class. It |
Afwillite is typically found in the veins of [[spurrite]] and it belongs to the nesosilicate sub-class. It has a[[monoclinic]] [[crystal system]], its space group is P2 and its point group is 2. |
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===Formation of afwillite=== |
===Formation of afwillite=== |
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Revision as of 20:34, 9 December 2007
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.
Afwillite is typically found in the veins of spurrite and it belongs to the nesosilicate sub-class. It has amonoclinic crystal 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.[1] Afwillite, as well as spurite, are formed through contact metamorphism of limestone.[2] Contact metamorphism is caused by the interaction of rock with heat and/or fluids from a nearby crystallizing silicate magma.[3]
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.[4] It has perfect cleavage parallel to its (101) and poor cleavage parallel to its (100).[5] 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.[1] Under a microscope afwillite optically looks like wollastonite, which is in the same family as afwillite.
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.[4] 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.[4] 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.[3]
Occurrence in concrete
Afwillite is enriched calcium silicate that is used in Portland cement. The cement gets its strength from the hydration of its di- and tri- calcium silicates.
References
- ^ a b Kusachi, I, Henmi, C. And Henmi K. (1989) Afwillite and jennite from Fuka, Okayama Prefecture. Japan. Miner J. 14, 279-292.
- ^ Barthemy, D. (2000) Afwillite Mineral Data, (http://webmineral.com/data/Afwillite.shtml)
- ^ a b Klein, C. and Dutrow, K. (2007) Manual of Mineral Science. 23rd Edition, 63, 596
- ^ a b c Malik, K. M. A & Jeffery J. W. (1976) A Re-investigation of the Structure of Afwillite. Acta Cryst. B32, 475
- ^ Megaw H. D. (1952). The structure of Afwilltie. Acta. 5, 477
External links
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