13.7.6: Paired Tetrahedral Silicates

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13.42.png — 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).

13.43.png — Figure 13.43: Crystals of lawsonite from Mendocino County, California

13.44.png — Figure 13.44: Cubes of gehlenite from Monte Monzoni, Italy

13.45.png — Figure 13.45: Zoisite from the Merelani Hills, Tanzania. The polished gemstone is about 0.5 cm wide.