Wollastonite structure

Spectra with narrow gates where the centers with a short decay time are emphasized enables us to detect broad bands at 794 nm with a decay time of 5 ps and broad band at 840 nm with a decay time of 190 ps (Fig. 4.42). These bands maybe ascribed to Cr luminescence centers, in addition to Cr with narrow -lines at 720 nm, detected by steady-state spectroscopy (Min ko et al. 1978). Wollastonite structure has three different types of six-coordinated calcium-oxygen groups, which enables the formation of several types of Cr " limiinescence centers. Nevertheless, luminescence of Cr " as result of Ca substitution has not been confidently found yet and another interpretation is also possible. For example, ions of V maybe considered, which have similar luminescence properties with Cr + and may substitute in Ca + sites. 

Pseudowollastonite (CaSiOs), the high temperature form of wollastonite, displays in its stmeture ealeium ion ehains easily removals by protons, as it was shown by Bailey and Reesman [86] in the study of the kinetic of dissolution of the wollastonite in H2O-CO2. system. This faet suggests the possibility of extraction of calcium ions, from wollastonite structure by protons in an appropriate medium, favouring therefore the precipitation and formation of a layer of HA on the surfaee of the material. 

Various additives and fillers may be employed. Calcium carbonate, talc, carbon black, titanium dioxide, and wollastonite are commonly used as fillers. Plasticizers are often utilized also. Plasticizers may reduce viscosity and may help adhesion to certain substrates. Thixotropes such as fumed silica, structured clays, precipitated silica, PVC powder, etc. can be added. Adhesion promoters, such as silane coupling agents, may also be used in the formulation [69]. 

Chain silicates, consisting of connected metasilicate units, (SiOg) " (Q ), and of an open structure. Wollastonite Ca(SiOg) reacted completely with poly(acrylic add), but the cement was much affected by water. 

Fig. 2.1 Configurations of the tetrahedral units and chain, double chain, and sheet structures in the silicate and aluminosilicate minerals. (A) Two-dimensional representation of a single silicate tetrahedron. (A ) Two-dimensional representation of an extended silicate chain. (B) Three-dimensional representations of single tetra-hedra in two orientations. The apexes of the tetrahedra point above or below the plane of the paper. (B ) Three-dimensional representations of extended silicate chains showing different orientations of the tetrahedra in two of the many possible configurations. Single chain pyroxenes (C), wollastonite (D), rhodonite (E). Double chains amphiboles (F). Sheets as found in the serpentines, micas, and clays (G).

Another pyroxenoid, bustamite, [(Mn,Ca,)3Si309], whose stmcture closely approximates that of wollastonite, has also been identified in fibrous form, but no detailed examination has been undertaken to check the possibility that a structural segregation similar to wollastonite exists in this mineral, and might contribute to the formation of fibers. 

Wollastonite is calciiun sihcate with a triclinic crystal system (P21). It has infinite-chain structure, with three tetrahedra per unit cell arranged parallel to y, this repeat unit consists of a pair of tetrahedra joined apex to apex as in the [Si07] group, alternating with a single tetrahedron with one edge parallel to the chain direction. Steady-state luminescence of wollastonite has been previously studied and luminescence of Mn, Fe and supposedly Cr has been proposed (Min ko et al. 1978). 

Calcium oxide is the main ingredient in conventional portland cements. Since limestone is the most abundant mineral in nature, it has been easy to produce portland cement at a low cost. The high solubility of calcium oxide makes it difficult to produce phosphate-based cements. However, calcium oxide can be converted to compounds such as silicates, aluminates, or even hydrophosphates, which then can be used in an acid-base reaction with phosphate, forming CBPCs. The cost of phosphates and conversion to the correct mineral forms add to the manufacturing cost, and hence calcium phosphate cements are more expensive than conventional cements. For this reason, their use has been largely limited to dental and other biomedical applications. Calcium phosphate cements have found application as structural materials, but only when wollastonite is used as an admixture in magnesium phosphate cements. Because calcium phosphates are also bone minerals, they are indispensable in biomaterial applications and hence form a class of useful CBPCs that cannot be substituted by any other. 

Compressive strength, psi (MPa) Fly ash, wollastonite 6000-12,000 (42-84) 4000 (28) Structural ceramic, waste management... 

Fracture toughness (MPay ) Wollastonite 0.3-0.7 =0.3 Range of structural materials applications... 

Stacking faults are a-boundaries for which a = 2xg R. (0gi-0g2) is zero for all g. In some structures, stacking faults and twins are closely related, and different regular sequences of these defects produce various polytypes. Wollastonite is a relatively simple example of such a structure, for which the stacking faults have been studied in some detail by TEM, both by their a-fringe contrast and in two-dimensional high-resolution lattice images. 

Figure 8.7. Facing page, (a) Structure of triclinic wollastonite, projected onto (001). Calcium atoms are represented by circles Si04 by tetrahedra. (b) Structure of one unit cell of a monoclinic wollastonite generated by a stacking fault with R = jfOlO] on the plane marked. Compare with (a), (c) Structure of one unit cell of monoclinic parawollastonite generated by twinning with a twin axis [010]. Comparison with (b) shows that this structure is identical to that generated by a single stacking fault in triclinic wollastonite. (From Hutchison and McLaren 1976,1977.)...

Because of their structural relationship, wollastonite and pturawolla-stonite are better described as polytypes rather than as polymorphs. Other polytypes are possible (Deer, Howie, and Zussraan 1963). For example, a new polytype would be formed if regular stacking faults occurred in every third triclinic unit cell, producing the packing sequence... 

Stacking disorder in wollastonite and its relationship to twinning and the structure of parawollastonite. Contrib. Mineral. Petrol., 61,11-13. 

Structure is used as a filler wollastonite CaSi03. The last natural silicate filler worthy of mention is perlite, which is manufactured from volcanic glassy stone by thermal expansion (see Section 5.3.6.3) and is utilized both in unground and ground form. 

Among the first applications of DAS and DOR were O studies of crystalline silicates, in which spectacular gains in resolution of up to two orders of magnitude were obtained (Chmelka et al. 1989, Mueller et al. 1991, Mueller et al. 1992). In the case of wollastonite, nine sites were resolved, in agreement with the site multiplicity expected from the crystal structure (Figure 6.4). The use of field-dependent data to deduce isotropic information from changes in the isotropic position has now been supplemented by MQ MAS NMR. In the second dimension of the 2D DAS and MQ data sets... 

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