The amphibole group includes a number of minerals, which have a closely related structure and chemical composition. The amphiboles contain hydroxyl group in the structure. The angle between two sets of cleavages is 124° and is characteristic of amphiboles. “Trimolite” (hydrous silicate of Ca and Mg), ”actinolite” (hydrous silicate of Ca, Mg, and Fe), and “hornblende” (hydrous silicate of Na, Ca, Fe, and Al) are the important members of amphibole group.
(1). Tremolites and Actinolites:
Chemical formula of tremolite is Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2, , and Actinolite is Ca2(Mg4.5–2.5Fe2+0.5–2.5)Si8O22(OH)2 . Physical properties of these two minerals is as under:
- Crystal System: Monoclinic
- Cleavage: Perfect
- Hardness: 5-6
- Sp.Gr: 3.0-3.3
- Luster: Vistreous and silky
- Colours: The colour of Tremolite is white or grey, while the colour of actinolite is green.
- Habit: tremolite is often bladed, and in rare cases in silky fibre, while habit of actinolite is radiating aggregate, and fibrous.
Occurrence of Tremolite and Actinolite: These minerals are commonly found in metamorphic rocks. Tremolite occurs in metamorphosed dolomitic limestones and actinolite is a characteristic mineral of the greenish facies of metamorphism.
Uses of Tremolite and Actinolite: The fibrous varieties of amphiboles are used as asbestos.
The name is used as a general or field term, to refer to a dark amphibole. Chemical formula of hornblende/dark amphibole is Ca 2(Mg,Fe,Al) 5(Al,Si) 😯 22(OH) 2. Its physical properties are following;
- Crystal System: Monoclinic
- Cleavage: Hornblende has a good prismatic cleavage.
- Hardness: Hornblend has a hardness of 5-6.
- Sp.Gr.: Specific gravity of hornblende is 3.0-3.4.
- Luster: Hornblende has a vitreous luster.
- Colours: Hornblende has a dark-green and black colours.
- Habit: The crystal of hornblende is six sided cross-section, massive and granular.
Occurrence of hornblende: Hornblende is an important rockforming mineral. It occurs commonly in syenites and diorites (coarse-grained igneous rocks). Hornblende constitutes a large part of the amphibole mineral.
(3). Uses of Amphiboles:
- As amphibole has a good cleavage, therefore the rocks containgin amphibole minerals are used as paving stone, and as a veneer on buildings. The rocks which contain amphibole are called amphibolite.
- Amphibolites (amphibole-containing rocks) are used as wall sheaths, which protect the building walls from environmental and chemical effects.
- Amphibolite are used as crushed stone: A crushed stone is used as a boulder for roads and building foundations.
- Anthophyllite, riebeckite, cummingtonite/grunerite series, and actinolite/tremolite series are the fibrous amphibole. The fibrous amphiboles are used as asbestos.
(4). Mineral Species of Amphibole class:
- Anthophyllite, (Fe,Mg)7Si8O22(OH)2
- Holmquistite, Li2Mg3Al2Si8O22(OH)2
- Ferrogedrite, Fe2+5Al4Si6O22(OH)2
- Tremolite, Ca2Mg5Si8O22(OH)2
- Actinolite, Ca2(Mg,Fe)5Si8O22(OH)2
- Cummingtonite, Fe2Mg5Si8O22(OH)2
- Grunerite, Fe7Si8O22(OH)2
- Hornblende, (K,Na)0-1(Ca,Na,Fe,Mg)2(Mg,Fe,Al)5(Al,Si)8O22(OH)2
- Glaucophane, Na2(Mg,Fe)3Al2Si8O22(OH)2
- Riebeckite (or Crocidolite), Na2Fe2+3Fe3+2Si8O22(OH)2
- Arfvedsonite, Na3Fe2+4Fe3+Si8O22(OH)2
- Richterite, Na2Ca(Mg,Fe)5Si8O22(OH)2
- Pargasite, NaCa2Mg3Fe2+Si6Al3O22(OH)2
- Winchite, (CaNa)Mg4(Al,Fe3+)Si8O22(OH)2
- Edenite, NaCa2Mg5(Si7Al)O22(OH)2
- Kaersutite, NaCa2(Mg3Ti4+Al)(Si6Al2)O22(OH)2