13.7.7: Isolated Tetrahedral Silicates

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13.46.jpg — Figure 13.46: The atomic arrangements in some isolated tetrahedral silicates

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.

13.47.png — Figure 13.47: Green titanite crystals on K-feldspar from near Meknes, Morocco

13.48.png — Figure 13.48: Brown zircon crystal from Pakistan. The specimen is 6 cm wide.

13.49.png — Figure 13.49: Topaz from northern Pakistan. The crystal is about 3 cm long.

13.50.png — Figure 13.50: Twinned staurolite from the Kola Penninsula, Russia

13.51.png — Figure 13.51: Brown monticellite with blue calcite from Crestmore, California

13.52.png — Figure 13.52: Andalusite crystals with classic dark *chiastolite* crosses caused by graphite inclusions

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).