Insular silicates

We proceed similarly to calculate the hardness of complex oxides and sulphides (Table 3.8), as well as insular silicates, both simple and complex. 

We need to digress briefly to examine the structures of some of the ions that are formed by the condensation of the nesosilicate ion, SiO/" (the word neso is from the Greek nesogaean meaning island. The nesosilicates are sometimes also referred to as insular silicates). Two SiOA ions can combine by sharing one oxygen to form the sorosUicate ion. This ion is shown below with two tetrahedra joined by one comer. 

VIII - 64 Cyclosilicates with rings with other anions and insular silicate groups VIII - 65 Inosilicate minerals... 

The affinity between the tetrahedrally arranged orthophosphate oxyanion, P04, and hexava-lently coordinated metal cations lends itself to a classification of phosphate minerals in a scheme similar to silicates (SiO -) framework, insular, chain, and layer phosphates. Examples of this scheme, advanced by Povarennykh (1972) and further elaborated by Lindsay Vlek (1977), include berlinite (AIPO4 framework) hydroxyapatite (insular) monetite (CaHP04, chain) and vivianite (Fe4(P04)2-2H2O, layer). 

Insular (group and spatial) silicates, like complex oxides, are considered to be minimally anisodesmic compounds, and the formula (3.8) allows fairly accurate calculation of their values. 

We calculate the hardness of simple insular and complex spatial silicates Zircon ZrSi04 am (Zr and Si tetravalent) ... 

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