Calcium hexagonal

Hydroxyapatite, Ca2Q(PO (OH)2, may be regarded as the parent member of a whole series of stmcturaHy related calcium phosphates that can be represented by the formula M2q(ZO X2, where M is a metal or H O" Z is P, As, Si, Ga, S, or Cr and X is OH, F, Cl, Br, 1/2 CO, etc. The apatite compounds all exhibit the same type of hexagonal crystal stmcture. Included are a series of naturally occurring minerals, synthetic salts, and precipitated hydroxyapatites. Highly substituted apatites such as FrancoHte, Ca2Q(PO (C02) (F,0H)2, are the principal component of phosphate rock used for the production of both wet-process and furnace-process phosphoric acid. 

Preparation. Hexagonal boron nitride can be prepared by heating boric oxide with ammonia, or by heating boric oxide, boric acid, or its salts with ammonium chloride, alkaU cyanides, or calcium cyanamide at atmospheric pressure. Elemental nitrogen does not react with boric oxide even in the presence of carbon, though it does react with elemental boron at high temperatures. Boron nitride obtained from the reaction of boron trichloride or boron trifluoride with ammonia is easily purified. 

Chlorination of lime slurried in strong NaOCl solution, followed by cooling to — 15°C, precipitates about 80% of the av CI2 as large hexagonal crystals of a triple salt Ca(OCl)2 NaOCl NaCl -12H20 [64147-46-2] (182). The recovered triple salt, free of much of the lime impurities, is treated with chlorinated lime slurry to produce neutral calcium hypochlorite dihydrate crystals [22464-76-2]. 

The dehydration and interconversion reactions of the various forms of calcium sulphate [dihydrate, hemihydrate (a and 3 pseudo morphs) and anhydrous salt (hexagonal and orthorhombic structures)] have been studied by Ball et al. [281,590,591] who compared their observations with the available rate and microscopic data. The important features of the complicated behaviour found are summarized in the scheme [281]... 

Figure 4.17. The binary phase diagrams of the magnesium alloy systems with the divalent metals ytterbium and calcium (Ca is a typical alkaline earth metal and Yb one of the divalent lanthanides). Notice, for this pair of metals, the close similarity of their alloy systems with Mg. The compounds YbMg2 and CaMg2 are isostructural, hexagonal hP12-MgZn2 type.

A single substance may crystallise in more than one of seven crystal systems, all of which differ in their lattice arrangement, and exhibit not only different basic shapes but also different physical properties. A substance capable of forming more than one different crystal is said to exhibit polymorphism, and the different forms are called polymorphs. Calcium carbonate, for example, has three polymorphs — calcite (hexagonal),... 

Calcium carbonate occurs in two forms— hexagonal crystal known as cal-cite, and orthorhombic form, aragonite. Calcite decomposes on heating at 825°C, aragonite melts at 1,339°C (at 102.5 atm). Density 2.71 g/cm (calcite), 2.83 g/cm3 (aragonite) insoluble in water (15mg/L at 25°C) Ksp 4.8x10 soluble in dilute mineral acids. 

Pure product is a colorless, hexagonal crystal or white powder. Commercial grade material may he grayish-hlack powder or lump (the color is due to presence of calcium carbide and other impurities) density 2.29 g/cm melts around 1,340°C sublimes around 1,150 to 1,200°C on rapid heating reacts with water. 

Fig. 2. Structure of fluorapatite. Projection on the (001) cristallographic plane, perpendicular to the c axis of the hexagonal structure. (Reproduced by permission of lUCrfrom Ref. [2]). Purple Calcium green Fluorine red Oxygen yellow Phosphorus. (See Colour Plate Section at the end of this book.)...

Fig. 1.19 Adsorption-desorption isotherms of calcium lignosulfonate on C3A hydrate (hexagonal phase) (Ramachandran).

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