13.7.7: Isolated Tetrahedral Silicates
- Page ID
- 18371
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Mineralogists often call isolated tetrahedral silicates island silicates because tetrahedra do not share oxygen. Island silicates contain tetrahedral and octahedral sites, and no sites large enough to hold alkalis and other large cations. Figure 13.46 shows atomic arrangements in some examples: titanite, olivine, and garnet.
Titanite (Figure 13.46a). contains octahedral Ca2+ alternating with (SiO4)4- tetrahedra and (TiO6)8- octahedra. In olivine, divalent octahedral cations (usually Fe2+ or Mg2+), occupying two slightly different-sized sites, link independent silicon (SiO4)4- tetrahedra (Figure 13.46b). In monticellite, CaMgSiO4 (a member of the olivine group), the larger octahedral site contains Ca2+. The garnet structure (Figure 13.46c) contains (SiO4)4- tetrahedra sharing oxygen with distorted octahedral and cubic sites. The octahedral site normally contains Al3+, but sometimes by Fe3+ or Cr3+. Ca2+, Mg2+, Fe2+, and sometimes Mn2+ occupy the cubic site.
The island silicate group contains many important minerals besides titanite, olivine, and garnet. These include zircon (ZrSiO4), andalusite (Al2SiO5), kyanite (Al2SiO5), sillimanite (Al2SiO5), topaz (Al2SiO4(F,OH)2), and staurolite (Fe2Al9(SiO4)4(O,OH)2). Photos below show some examples.
Other photos of island silicates can be found in Chapters 3, 4, 6, 8, and 10: olivine (Figures 6.1 and 6.20), garnet (Figures 3.6, 8.10, 8.21, 8.23, 8.30, 8.44, and 10.48), kyanite (Figures 8.46, and 8.51), and staurolite (Figures 4.40, and 8.45).