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Kathy Feick
The name amphibole is derived from the Greek work amphiboles, meaning “ambiguous”. The amphibole group is extensive and complex. It is presently divided into several sub-groups and remains under study. The basic structure of an amphibole is a double-chain of tetrahedral.
Occurrence:
Amphiboles have a wide range of chemical substitutions. Their chemical composition and general characteristics are very similar to the pyroxene group. They are an important constituent in a variety of plutonic and volcanic igneous rocks. Igneous amphiboles intermediate in composition are referred to as hornblende. Amphiboles can also crystallize in metamorphic rocks, especially those derived from mafic igneous rocks and siliceous dolomites. Finally, because of their relative instability to chemical weathering at the Earth’s surface, amphiboles are not present in large amounts in sedimentary rocks.
Amphiboles pieces image source: http://www.pitt.edu/%7Ecejones/GeoImages/1Minerals/1IgneousMineralz/Amphiboles.html
Can you see the 60 and 120 degree cleavage?
Identification:
Typically, amphiboles form as long prismatic crystals, radiating sprays and fibrous aggregates. They are generally dark coloured though their colours can range from colourless to white, green, brown, black, blue or lavender. This property is related to composition, particularly iron content. Finally, an especially important diagnostic property for amphiboles is their cleavage. They have two directions of cleavage at about 60 and 120 degrees.
Minerals in this group:
There are approximately 76 chemically defined end-member amphiboles. The following list is contains only the most common amphiboles.
Orthorhombic series
- Anthophyllite
Monoclinic series
- Tremolite
- Actinolite
- Cummingtonite
- Grunerite
- Hornblende
- Glaucophane
- Riebeckite
- Arfvedsinite
- Crocidolite
- Richterite
- Paragasite