Afwillite is a

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water ...

nesosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually cons ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...

with formula Ca₃(SiO₃OH)₂·2H₂O. It occurs as glassy, colorless to white prismatic

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...

crystals. Its

Mohs scale The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the ...

hardness is between 3 and 4. It occurs as an alteration mineral in

contact metamorphism Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

. It occurs in association with

apophyllite The name apophyllite refers to a specific group of phyllosilicates, a class of minerals. Originally, the group name referred to a specific mineral, but was redefined in 1978 to stand for a class of minerals of similar chemical makeup that compris ...

natrolite Natrolite is a tectosilicate mineral species belonging to the zeolite group. It is a hydrated sodium and aluminium silicate with the formula . The type locality is Hohentwiel, Hegau, Germany. It was named natrolite by Martin Heinrich Klap ...

thaumasite Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedral configuration, with chemical formula Ca3 Si(O H)6( C O3)( SO4)·12 H2O, also sometimes more simply written as CaSiO3·CaCO3·CaSO4·15H2O. It occurs as colorless ...

, merwinite, spurrite,

gehlenite Gehlenite, (Ca2Al lSiO7, is a sorosilicate, Al-rich endmember of the melilite complete solid solution series with akermanite.Deer et al., 1993 The type locality is in the Monzoni Mountains, Fassa Valley in Trentino in Italy, and is named after ...

ettringite Ettringite is a hydrous calcium aluminium sulfate mineral with formula: . It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part o ...

portlandite Portlandite is a hydroxide-bearing mineral typically included in the oxide mineral class. It is the naturally occurring form of calcium hydroxide (Ca(OH)2) and the calcium analogue of brucite (Mg(OH)2). Occurrence Portlandite occurs in a variety ...

, hillebrandite, foshagite,

brucite Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg( OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and ...

and

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scra ...

. It was first described in 1925 for an occurrence in the Dutoitspan Mine,

Kimberley, South Africa Kimberley is the capital and largest city of the Northern Cape province of South Africa. It is located approximately 110 km east of the confluence of the Vaal and Orange Rivers. The city has considerable historical significance due to its ...

and was named for Alpheus Fuller Williams (1874–1953), a past official of the

De Beers De Beers Group is an international corporation that specializes in diamond mining, diamond exploitation, diamond retail, diamond trading and industrial diamond manufacturing sectors. The company is active in open-pit, large-scale alluvial and ...

diamond company. Afwillite is typically found in veins of spurrite and it belongs to the nesosilicate sub-class. It is monoclinic, its space group is P2 and its point group is 2.

Formation of afwillite

It is suggested that afwillite forms in fractured veins of the mineral spurrite.

Jennite Jennite is a calcium silicate hydrate mineral of general chemical formula: Ca9Si6O18(OH)6·8H2O. Jennite occurs as an alteration mineral in metamorphosed limestone and skarn. It typically occurs as vein and open space fillings as a late miner ...

, afwillite, oyelite and

are all minerals that form in layers within spurrite veins. It appears that afwillite, as well as calcite, forms from precipitated fluids. Jennite is actually an alteration of afwillite, but both formed from calcium silicates through hydration. Laboratory studies determined that afwillite forms at a temperature below , usually around 100 °C.Kusachi, I, Henmi, C. And Henmi K. (1989) Afwillite and jennite from Fuka, Okayama Prefecture. Japan. Miner J. 14, 279-292. Afwillite and spurrite are formed through

of limestone.Barthemy, D. (2000) Afwillite Mineral Data, (http://webmineral.com/data/Afwillite.shtml) Contact metamorphism is caused by the interaction of rock with heat and/or fluids from a nearby crystallizing silicate

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natura ...

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

Structure and properties

Afwillite has a complex monoclinic structure, and the silicon tetrahedra in the crystal structure are held together by

hydrogen bonds In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

.Malik, K. M. A & Jeffery J. W. (1976) A re-investigation of the structure of afwillite. Acta Crystallographica, B32, 475. It has perfect cleavage parallel to its (101) and poor cleavage parallel to its (100) faces.Megaw H. D. (1952). The structure of afwillite. Acta Crystallographica, 5, 477. It is biaxial and its 2V angle, the measurement from one optical axis to the other optical axis, is 50 – 56 degrees. When viewed under crossed polarizers in a

petrographic microscope A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology wh ...

, it displays first-order orange colors, giving a maximum birefringence of 0.0167 (determined by using the Michel- Levy chart). Afwillite is optically positive. Additionally, it has a

prismatic An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of opti ...

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macr ...

habit. Under a microscope afwillite looks like

wollastonite Wollastonite is a calcium inosilicate mineral ( Ca Si O3) that may contain small amounts of iron, magnesium, and manganese substituting for calcium. It is usually white. It forms when impure limestone or dolomite is subjected to high temperature ...

, which is in the same family as afwillite. Afwillite 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 Miller indices form a notation system in crystallography for lattice planes in crystal (Bravais) lattices. In particular, a family of lattice planes of a given (direct) Bravais lattice is determined by three integers ''h'', ''k'', and '' ...

101faces. The sheets are bonded together by

and are all connected by Ca-Si-O bonds (Malik and Jeffery, 1976). Each calcium atom is in 6-fold octahedral coordination with the oxygen, and the silicon is in 4-fold tetrahedral coordination around the oxygen. Around each silicon there is one OH group and there are three oxygens that neighbor them. The silicon tetrahedra are arranged so that they share an edge with calcium(1), and silicon(2) shares edges with the calcium(2) and calcium(3) polyhedral. The silicon tetrahedra are held together by the OH group and

hydrogen bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing ...

occurs between the hydrogen in the OH and the silicon tetrahedra.

Hydrogen bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing ...

is caused because the positive ion, hydrogen, is attracted to negatively charge ions which, in this case, are the silicon tetrahedra.

Occurrence in concrete

Afwillite is one of the

calcium silicate hydrate Calcium silicate hydrate (or C-S-H) is the main product of the hydration of Portland cement and is primarily responsible for the strength in cement based materials (e.g. concrete). Preparation When water is added to cement, each of the compounds u ...

s (C-S-H) that form when

Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th cen ...

sets to form hardened cement paste (HCP) in

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...

.http://www.minersoc.org/pages/Archive-MM/Volume_29/29-216-838.pdf {{Webarchive, url=https://web.archive.org/web/20160527122357/http://www.minersoc.org/pages/Archive-MM/Volume_29/29-216-838.pdf , date=2016-05-27 Moody, K. M., 1952, ''The thermal decomposition of afwillite,'' The Mineralogical Society The cement gets its strength from the hydration of tri- and di- calcium silicates (C₃S and C₂S) present in the clinker.

References

Calcium minerals Cement Concrete Nesosilicates Hydrates Geology of Riverside County, California Crestmore Heights, California Monoclinic minerals Minerals in space group 9

Formation of afwillite

Structure and properties

Occurrence in concrete

See also

References