Amphiboles

AMPHIBOLES (from the Greek word "amphibolos", which means "ambiguous", "uncertain"; according to the diverse composition of the forms of isolation * EN: amphiboles; DE: Amphibole; FR: amphiboles; ES: anfiboles; RU: амфиболы) is the large group of the rock-forming minerals of the class of silicates with the general formula (X,Y)7-8(Z4O11)2(OH,F,Cl)2, where

  • X - Na, K, Ca;
  • Y - Al, Fe3 +, Fe2 +, Mg, sometimes Mn, Ti, Cr, Li;
  • Z - Si, Al (where Al:Si > 1:3).

Amphiboles are the band silicates and aluminosilicates. The foundations of their crystal structure are the dual chains of the [(Al,Si)04] tetrahedra, which are tightly bound by cations and OH-, F- and Cl- ions, which are obligatory for amphibolites. As the crystal structure of amphiboles tolerates the diverse isomorphic substitutions, there are distinguished many minerals, which hold the intermediate place according to the composition among the main representatives of the family. Most amphibolites crystallize in the monoclinic crystal system (all Ca-, Na-, and part of Mg-, Fe-Amphiboles), but there are also known the less common rhombic Mg-, Fe-, and Li-Amphiboles.

They distinguish among the amphibolites, according to their composition, several isomorphous series: anthophyllite-gedrite (rhombic Mg-, Fe-Amphiboles), cummingtonite-grunerite (monoclinic, Mg-, Fe-Amphiboles), tremolite-actinolite (monoclinic, Ca-Amphiboles), hornblende (Ca-, Na-Amphiboles), richterite and arfvedsonite (Na-, Ca-Amphiboles), glaucophane-riebeckite (Na-Amphiboles), basaltic hornblende (with Fe3+), holmquistite (Li-Amphiboles). Amphibolites, which are rich in Na, are named alkaline. The colour of amphibolites is black, green of various shades, less often white (tremolite, edenite) or blue (glaucophane), to dark purple (holmquistite). They form elongated prisms, columnar, needle-like, or thin-fibrous (amphibole asbestos) aggregates. The characteristical feature of all amphibolites is the perfect cleavage along the prism with the angle between the planes of approximately 124 degrees, which distinguishes amphibolites from the externally similar pyroxenes. The hardness is 5-6.5. The density is 2850-3600 kilograms per cubic metre.

Amphibolites are widespread rock-forming minerals of many erupted and metamorphic rocks (granodiorites, diorites, syenites, certain gabbros, andesites and basalts, hornblendites, amphibolites, green schists, hornfels, skarns, and others); alkaline amphibolites may be found within the alkaline and nepheline syenites, alkali granites and metasomatites, ferruginous quartzites, carbonatites, albitites. Amphibole asbestos (so-called blue asbestos, and other fibrous varieties of alkaline amphibolites and hornblende) have the technical usage. The dense hidden-crystalline aggregate of actinolite or tremolite with entangled-fibrous microstructure, namely, Jade, is jewellery and ornamental stone. The silicified fibrous alkaline amphiboles (crocidolite), including the weathered with goethite ones, are the valuable jewellery and ornamental stones (cat's, hawk's, tiger's eye). The ferruginous amphibolites within certain skarns contain up to 0.35-0.4% Sn (which isomorphically substitutes Fe3+), and may be considered as prospective tin ore.