Island silicates

When the O/Si ratio is 4, the structural units are isolated (Si04) tetrahedra. These units are connected by positive ions. The resulting structure gives rise to what is called an island silicate. In garnets (Mg,Fe %Mn,Ca)3(Cr,Al, Fe )2(Si04)3 and olivines (Mg,Fe )2(Si04), (Si04) becomes the anion. These anions are bonded ionically by the metallic cations. 

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Island silicates containing discrete ions. Orthosilicates, SiO " (Q ) pyrosilicates, Si O " (Q ) and ring silicates, SigO,", SieOir (Q ). 

Fig. 5.18. The Si L2 3 x-ray emission spectra of various silicates, as shown, and of quartz. In terms of structural silicate groups (1-7) are island silicates, (8-9) ring silicates, (10-U) single-chain silicates, (12-15) double-chain silicates, (16-19) sheet silicatesj (20-22) framework silicates (after Dikov et al., 1976 reproduced with the publisher s permission).

There is not much resistance to weathering in these minerals because of the relative lack of Si—O—Si bonding, especially in island silicates such as olivine. Layer silicate minerals rich in Mg (e.g., trioctahedral smectites, chlorite, serpentine) may form from the siliceous residue if leaching does not deplete in the weathering zone. 

Pyrosilicates (Si207)6- - soro-silicates, Disilicates Two tetrahedral units are joined by sharing the O at one comer, thus giving (Si207)6-. The structure possessed by them are called island silicates stmcture(Fig)... 

Nesosilicates and sorosilicates are sometimes grouped together under the name island silicates (Rostov [1954]). The way in which individual tetrahedra may link to give these formulas is illustrated in Figure 1. It should be mentioned at this point that AP may frequently substitute for Si in tetrahedra, so that instead of, e.g., SiO, one may have AlSi30i2. The nomenclature of Strunz [1957] is employed in the following discussion. 

Almost all rocks contain some aluminum in the form of aluminum silicate minerals found in clays, feldspars, and micas. Today, bauxite is the major ore for the source of aluminum metal. Bauxite was formed eons ago by the natural chemical reaction of water, which then formed aluminum hydroxides. In addition to the United States, Jamaica and other Caribbean islands are the major sources of bauxite. Bauxite deposits are found in many countries, but not all are of high concentration. 

This is divided into a western, a central and an eastern sector. The western Aeolian arc (Alicudi, Filicudi, Salina) consists of calc-alkaline rocks with typical island arc signatures. Mafic and intermediate rocks dominate the volcanic sequence, with minor silicic volcanics. The central islands (Vul-cano and Lipari) are dominated by calc-alkaline to shoshonitic mafic to silicic rocks mafic rocks from this sector show isotopic compositions and incompatible trace element ratios similar to the western islands. The eastern arc (Panarea and Stromboli) consists of calc-alkaline to potassic alkaline rocks. Stromboli shows geochemical and isotopic signatures akin to the Neapolitan volcanoes. The Island of Panarea, located between Stromboli and Lipari, has intermediate characteristics between these two volcanoes (Calanchi et al. 2002a). 

Silicic volcanics occur as lavas at San Vincenzo, Roccastrada, Monte A-miata, Monti Cimini and Tolfa-Manziana-Cerite complex. Pyroclastic rocks are scarce or absent, and a few ignimbrites only occur at Monti Cimini and Cerite complex. Silicic intrusions crop out in the islands of Elba, Montecristo and Giglio, and at Campiglia and Gavorrano in southern Tuscany. Other granitoid bodies occur as seamounts in the northern Tyrrhenian Sea (e.g. Vercelli Barbieri et al. 1986) and as hidden intrusions in several places of southern Tuscany (e.g. Franceschini et al. 2000 Poli et al. 2003). 

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