Dahllite

McConnell, D. (1960). The crystal chemistry of dahllite. American Mineralogist 45 209-216. 

The biologically most relevant calcium phosphates are dahllite [carbonate hydroxyapatite, (Na,Ca)10(PO4,CO3)6(OH)2] francolite [carbonate fluoroapatite] hydroxyapatite [Ca10(PO4)6(OH)2]... 

About twenty different skeletal minerals are reported from organisms7,8 however, only four are common (1) aragonite, (2) calcite, (3) dahllite = carbonate hydroxyapatite, and (4) opal. The remaining minerals depicted in Fig. 1 are either trace constituents or occur only in a few isolated species. It is for this reason that the article concentrates on carbonate, phosphate and silica deposition in plants and animals. For reviews on general aspects of biomineralization and discussions on individual taxonomic classes see Ref.9-47 ... 

Of the four prominent pure end-member phosphate species of the apatite group only dahllite (space group C63/m) is a common biomineral28. This carbonate apatite is a distinct variety of apatite rather than a mixture of CaC03 and hydroxyapatite. Expressed as oxides, the mineral content of bovine cortical bone (dry fat-free material) has the following chemical composition112 ... 

The nature of mineral phases present in bone, dentin, enamel and other phosphatic tissues, and their mode of formation have been subjects of lively discussions among health scientists and crystallographers. Bioscientists most commonly accept the viewpoint that the inorganic phase of bones or teeth is principally hydroxyapatite, Caio(P04)6(OH)2, and deviation in Ca/P ratio from common hydroxyapatite (Ca/P = 1.667) observed in mineralized tissues is explained by the presence of amorphous phosphates. In contrast, many crystallographers favor the idea of carbonate apatite, i.e. dahllite, as the major crystalline phase in biophosphates and they doubt the existence of amorphous phases. The topic has been reviewed14,15,22,28, 37,44,47,348-358) no common consent has yet been reached. In the following an attempt is made to at least coordinate the controversial findings. 

The most straight-forward possibility is one in which the mineral phase consists only of dahllite and the presence of amorphous inorganic solids is excluded28- 3S6- 363. ... 

Figure 1.4. (a) Scanning electron micrograph of a fractured cross section of a mineralized tooth from the radula in Fig. 1.1. (b) Enlargement of the upper part of the same fractured cross section. The magnetite layer (M) lines the interior surface. The thick dahllite layer (D) is on the exterior. The thin lepidocrocite layer (L) is sandwiched between the two. C, organic sheath and cell remnants. 

Ein Carbonat-Apatit wurde erstmals 1822 von Broegger et al. aus einem skandinavischen Vorkommen beschrieben und Dahllit benannt (12). Mit Dahllit werden heute im besonderen Hydroxylcarbonatapatite Ca5[0H (P04, C03, OH )3] bezeichnet, wahrend fluoridreichere Formen Cag[(OH, F) (P04, C03, OH )3] Francolith oder Stafielit benannt werden. 

Mineralvorkommen von Dahllit u.a. enthalten oft wesentlich mehr Carbonat (bis zu 10%) sie mussen daher neben dem in der Apatitstruktur gehaltenen Carbonat noch Calciumcarbonat in einer Separat-phase enthalten, welche aber durch amorphes Calciumcarbonatphosphat zu beschreiben ist. 

The most common apatite is Ca5(P04)30H and is called hydroxyapatite. Other forms include chloroapatite (Ca5(P04)3Cl), fluoroapatite (Ca5(P04)3F) and carbonate apatite or dahllite (Ca5(P04)3C03). These minerals are in pure forms, but it is also possible to generate them by partial replacement of one anion by another or one cation by another. For example, Ca may be replaced by Pb by ionic substitution, yielding pyromorphites [Pb5(P04)3(0H,Cl,F)]. As we shall see in Chapter 16, this mineral is very important in stabilizing the hazardous metal Pb. Also as discussed in Chapter 2 and shall be seen in later chapters, Mg-based CBPCs have many applications, and hence minerals such as Mg5(P04)3(0H,Cl,F) are also very common. 

Brown and Chow [7] have listed the important calcium phosphate compounds arranged in the order of increasing basicity (Ca to P ratio). This list is reproduced in Table 13.5 along with a short form notation for each compound in the first column. We have also included in the list an additional mineral, dahllite, recently synthesized by Constantz et al [14], because this mineral is an important compound found in bones. 

Recently, Constantz et al. [14] reacted a mixture of monocalcium phosphate monohydrate (Ca(H2P04)2-H20), a-tricalcium phosphate (Ca3(P04)2), and calcium carbonate (CaCOa) with a solution of trisodium phosphate (Na3P04) to produce dahllite with the stoichiometric formula. 

Typically, bone has a solid outer portion called cortical bone and a porous inner part called cancellous bone. The amounts of each vary with location in the body. The cortical bone is a ceramic containing calcium compounds and viscous liquids, a protein called collagen , and an organic polymer. In addition to HAP, bone consists of calcium carbonate and calcium phosphate. HAP is 69 wt.% of total calcium phosphate compounds [4]. Part of the Ca in these compounds is substituted by Na, K, Mg, and Sr. Hydroxyl ions in the HAP are also substituted by F, CO3, or Cl, which makes the apatite a fluoroapatite, dahllite or chloroapatite, respectively. These substitutions are considered to play significant roles in the structure and mechanical properties of bones. 

There have been significant advances in CBPC-based biomaterials in the last few years, particularly Ceramicrete-based dental cement and dahllite-based bioceramics. 

Constantz et al. [9,10], however, have succeeded in producing dahllite-based bioceramic with the stoichiometric formula... 

In clinical tests the dahllite-based material was injected as an implant for internal stabilization of a wrist fracture. The ingredients were mixed to form a slurry that was injected in the patient s body. The paste set within minutes under physiological conditions. The material was allowed to harden in situ before a cast was applied. The healing process was more rapid compared to conventional techniques. The new biomaterial resisted compressive forces from the musculature and the wrist healed faster than indicated by historical controls. 

The high brilliance X-ray diffraction pattern of the new biomaterial was compared with that of the cortical bone of a rabbit and also sintered HAP. The dahllite-based biomaterial had a diffraction pattern very similar to that of the bone, while sintered HAP was highly crystalline. The crystallite sizes of bone as well as their biomaterial were very similar, equal to an average size of 5 nm. Unlike bone, however, the crystallites in the biomaterial oriented unidirectionally because they were rapidly formed in the absence of an organic matrix. The density of the new biomaterial was found to be 1.3 g cm , and the average pore diameter was 30 nm. These measurements indicate that both crystallite size and the porosity are at the nanoscale and result in broad peaks in the X-ray diffraction pattern. Bigger crystallites would have yielded sharper peaks in the sintered HAP. 

Carbonates Phosphates Silica Calcite Aragonite Vaterite Monohydrocalcite Amorphous Dahllite Francolite Amorphous calcium phosphate hydrogel Amorphous ferric phosphate hydrogel Opal Iron oxides Sulfates Halides Oxalates Magnetite Goethite Lepidocrocite Amorphous hydrates Celestite Barite Gypsum Fluorite Weddellite Whewellite... 

From a cave inhabited by humans at Mount Carmel, Israel, montgomeryite and crandallite have been reported (Goldberg and Nathan, 1975), in addition to dahllite and what is reported to be hydroxyapatite — said to be almost devoid of CO2 although no analysis is given. This represents a mixed type of deposit in which calcium is derived from limestone and aluminium from clay. 

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