Amphibole

Amphibole (/ˈæmfəboʊl/ AM-fə-bohl) is a group of inosilicate minerals, forming prism or needlelike crystals,[1] composed of double chain SiO4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.

Each I-beam is bonded to its neighbor by additional metal ions to form the complete crystal structure.

The group was so named by Haüy in allusion to the protean variety, in composition and appearance, assumed by its minerals.

Ferrous iron usually substitutes freely for magnesium in amphiboles to form continuous solid solution series between magnesium-rich and iron-rich endmembers.

[18] In addition, the orthoamphiboles, anthophyllite and gedrite, which differ in their aluminium content, form a continuous solid solution at elevated temperature.

In addition, titanium, manganese, or chromium can substitute for some of the cations and oxygen, fluorine, or chlorine for some of the hydroxide.

A miscibility gap exists at lower temperatures, and, as a result, hornblende often contains exsolution lamellae of grunerite.

[21] On account of the wide variations in chemical composition, the different members vary considerably in properties and general appearance.

Anthophyllite occurs as brownish, fibrous or lamellar masses with hornblende in mica-schist at Kongsberg in Norway and some other localities.

An aluminous related species is known as gedrite and a deep green Russian variety containing little iron as kupfferite.

[22] Actinolite is an important and common member of the monoclinic series, forming radiating groups of acicular crystals of a bright green or greyish-green color.

The name (from Greek ἀκτίς, ἀκτῖνος/aktís, aktînos, a 'ray' and λίθος/líthos, a 'stone') is a translation of the old German word Strahlstein (radiated stone).

The latter two are dark green minerals, which occur as original constituents of igneous rocks rich in sodium, such as nepheline-syenite and phonolite.