13.7.4: Double Chain Silicates (Amphiboles)

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13.37.png — Figure 13.37: View down the c axis in tremolite

Figure 13.37 shows atomic arrangement in tremolite, an amphibole. Drawing a is a view down the chain direction (c axis). Drawing b shows a top view of the double tetrahedral chain. The tetrahedra generally contain Si⁴⁺ and sometimes lesser amounts of Al³⁺.

Like the pyroxenes, chains in amphiboles point up and down. But in contrast with pyroxenes, amphiboles are double-chain silicates with four different-sized octahedral sites (and seven octahedral sites in all) between apical oxygens. In Figure 13.37, we labeled the sites M1 through M4. Because they contain four different octahedral sites, each characterized by slightly different bonding, amphiboles generally have more complicated chemistries than pyroxenes. Nonetheless, the two mineral structures share many characteristics.

In amphiboles, the two M4 sites (orange) are larger than the other octahedral sites and may hold Ca²⁺ or Na^+. Any of the octahedral sites may contain Mg²⁺, Fe²⁺, Al³⁺, sometimes Ti⁴⁺, and other cations of similar size and charge. Like the pyroxenes, amphiboles may be orthorhombic or monoclinic. Twinning, both simple and complex, is common in amphiboles.

Tremolite, Ca₂Mg₅Si₈O₂₂(OH)₂ has relatively simple chemistry, similar to that of diopside. But, we can see a large hole just below the center of the tremolite model in Figure 13.37a. This site, called the A site, is vacant in tremolite and other calcic amphiboles. It has 10-12 fold coordination. In hornblende, the most common amphibole, this large interlayer site usually contains some K⁺ (somewhat like the K⁺ between layers in micas) and sometimes Na⁺. So, unlike pyroxenes, amphiboles can accommodate large cations.

Hornblende chemistry is highly variable; we might write a simplified formula as (K,Na)_0-1(Ca,Na,Mg)₂(Mg,Fe,Al)₅(Si,Al)₈O₂₂(OH)₂. The parentheses (from left to right in this formula) group elements that occupy the A site, the large octahedral site (M4), the smaller octahedral sites (M1 through M3), and the tetrahedral site. The subscript 0-1 tells us that the A site may not be fully occupied by K⁺ and Na⁺.

The photos below show two examples of amphiboles. Additional photos of amphiboles are in Chapter 6: anthophyllite (Figure 6.83), actinolite (Figure 6.84), and hornblende (Figure 6.86).

13.38.png — Figure 13.38: Tremolite from Campolungo, Switzerland. The sample is 14 cm across.

13.39.png — Figure 13.39: Hornblende from near Parry Sound, Ontario, Canada. The specimen is 6 cm tall.