Fibrous Silicates and Wollastonite

There are also naturally occurring fibrous silicate minerals. The most commercially important of these today is Wollastonite. It has the chemical formula CaSiOj. Wollastonite particles are shown in Fig. 1.16. Asbestos, now largely banned as a health hazard, is also a fibrous silicate. Wollastonite is obtained by mining or reaction between calcite and silica at temperatures higher than 450 °C  

The crystal structure of Wollastonite has been investigated using X-ray diffraction [93,95]. It is found that Wollastonite has two different crystallographic forms. The primary crystalline form, which is found in massive natural deposits, has a triclinic unit cell and was found by Peacock [96] to be  

This crystal structure is shown in Fig. 1.17. There is a second form known as Parawollastonite , which occurs chiefly in limestone blocks ejected from volcanoes. This has a monoclinic crystal structures with unit cell dimensions  

The two unit cells are very similar. The relationship between triclinic Wollastonite and monoclinic Parawollastonite is that, when in projection along the c-axis of the monoclinic unit cell, alternate layers of the two triclinic orientations form the monocKnic unit cell. 

Commercial Wollastonites have aspect ratio ranges up to 20 1 with an equivalent spherical diameter distribution ranges from 0.3 to 40 pm. 

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