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13.7.6: Paired Tetrahedral Silicates

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    Figure 13.42: The atomic arrangement in zoisite

    Lawsonite, a rare mineral found in blueschists, and the melilite minerals åkermanite and gehlenite (also rare) are perhaps the best examples of paired tetrahedral silicates. The paired tetrahedra result in Si2O7 groups. Other minerals, commonly grouped with lawsonite and the melilites, contain some paired and some unpaired tetrahedra. Zoisite, vesuvianite, and epidote, for example, contain both SiO4 and Si2O7 groups. Figure 13.42 shows the atomic arrangement in zoisite. Both paired SiO4 tetrahedra and isolated SiO4 tetrahedra are present. The structure also contains AlO6 octahedra (red); some of the Al-octahedra share edges. Ca2+ (orange) occupies large octahedral sites between silicon tetrahedra and aluminum octahedra.

    Figure 13.43 shows lawsonite from the blueschist terrane of northern California. Lawsonite is similar in many ways to a high-pressure form of anorthite. The photo in Figure 13.44 is gehlenite from near Bolzano, Italy. Figure 13.45 is a gemmy blue variety of zoisite called tanzanite. The raw stone in the top of photo is an imperfect orthorhombic prism; a cut and polished tanzanite gemstone is in the bottom of the photo.

    Additional photos of paired tetrahedral silicates are in Chapter 6 and Ch 8: epidote (Figure 6.91), tanzanite (Figure 6.92), lawsonite (Figure 6.93), and a spectacular photo of green zoisite with red corundum and black hornblende (Figure 8.1).

    Figure 13.43: Crystals of lawsonite from Mendocino County, California
    Figure 13.44: Cubes of gehlenite from Monte Monzoni, Italy
    Figure 13.45: Zoisite from the Merelani Hills, Tanzania. The polished gemstone is about 0.5 cm wide.

    This page titled 13.7.6: Paired Tetrahedral Silicates is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Dexter Perkins via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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