Amphiboles

Amphiboles 

a group of widely distributed rock-forming crystals consisting of oxysilicate compounds of magnesium, iron, calcium, less often aluminum, and alkalies; the crystals contain water, fluorine, and sometimes chlorine. Amphiboles are components of many igneous and metamorphic mountain rocks (hornblende gabbro, diorite, amphibolite, and others) and occasionally are the only constituent (hornblendite, amphibolitic shales). The amphiboles are classified as a group on the basis of their crystallographic properties. Amphiboles crystallize in monoclinic and rhombic systems, forming prismatic crystals. The basis of the crystalline structure of amphiboles, as in the case of all silicates, is oxysilicate tetrahedrons interconnected by two or three common oxygen ions into a continuous band (see Figure 1). The basic grouping has the composition [Si₄O₁₁]; the presence of (OH)₂, fluorine, or chlorine is required for every such radical in the structure. The color of amphiboles depends on their iron content; it may vary from colorless through green to black. The hardness of these minerals on the mineralogical scale is 5–6, and their density is from 2,800 to 3,460 kg/m³.

Figure 1. Crystalline structure of amphiboles

With regard to symmetry and chemical composition, the amphiboles are divided into a number of mineral groups comprising a large number of varieties which arise from wide isomorphic miscibility. Among magnesium-iron amphiboles are distinguished two varieties, rhombic (the anthophyllite-gedrite group) and monoclinic (the cummingtonite-grünerite group). Amphiboles of these two groups are encountered relatively rarely and only in metamorphic rocks. Calcium-containing amphiboles are known in monoclinic varieties. Among them are distinguished the actinolite group of alumina-free amphiboles found in metamorphic rocks, and a very large group of aluminocalcium amphiboles—the hornblendes, which are found both in magmatic and metamorphic rocks. Specific varieties are basaltic hornblende, found in effusive rocks, and alkaline hornblendes, usually part of alkali rocks. Alkaline sodium amphiboles are monoclinic (glaucophane, riebeckite, arfvedsonite, and others) and are characteristic of many metamorphic rocks and metasomatic formations. Upon deterioration or weathering, amphiboles are converted into chlorites, epidote, montmorillonitic clayey products, carbonates, hydrous ferric oxides, and silica. Fine-fibered varieties of amphiboles, known as amphibolitic asbestos, are widely used because of their chemical and thermal stability.

REFERENCES

Betekhtin, A. G. Kurs mineralogii, 2nd ed. Moscow, 1956.
Bragg, W. L., and G. Claringbull. Kristallicheskaia struktura mineralov. Moscow, 1967. (Translated from English.)

G. P. BARSANOV and V. P. PETROV