AMPHIBOLE, an important group of rock-forming min erals very similar in chemical composition and general char acters to the pyroxenes and, like them, falling into three series according to the system of crystallization (from the Gr. aµ 01.43oXos, "ambiguous") . They differ from the pyroxenes, how ever, in having an angle between the prismatic cleavages of 56° instead of 8 7 ° ; they are specifically lighter than the correspond ing pyroxenes, and in their optical characters they are distin guished by a stronger pleochroism and by a smaller angle of extinction on the plane of symmetry.
The amphiboles in their composition conform in general to the metasilicate formula R"SiO3, and are usually classified as follows :— Though, as already stated, there is a close similarity in com position between the amphiboles and pyroxenes, the former are the more complicated and their nature less understood. Most amphiboles contain some alkalis, fluorine and water of constitu tion and do not with few exceptions crystallize from dry melts. Rhombic amphibole of composition (kupfferite) has been obtained as an unstable phase by-rapid cooling of a MgSiO, melt. The natural rhombic amphiboles always contain water and usually some alkali. Gedrite may contain up to 2 2 % The cummingtonite-grunerite group of monoclinic amphiboles appears to form a solid solution series. The magnesian end mem ber has been prepared by rapid cooling of a melt and also by subjecting kupfferite in the presence of water to a tem perature of 3 7 5 7 5 ° C. Cummingtonite occurs together with anthophyllite in anthophyllite schists and in parallel intergrowth with the latter mineral or with gedrite in pneumatolytically meta morphosed sediments as in Finland and Cornwall. Griinerite occurs in association with garnet and fayalite in the rocks known as eulysites and is a prominent constituent of the altered iron bearing rocks of the Lake Superior region.
A lime-magnesia amphibole of the composition has been prepared by rapid cooling of a melt of the composition 85% diopside 15 % but in nature an amphibole of this composition is unknown. The sesquioxide-free amphiboles, trem olite and actinolite are most characteristic of metamorphosed calcareous sediments both thermal and dynamic, while in the metamorphosed igneous rocks—such as hornblende schists, the sesquioxide—rich hornblende is more common.
The hornblendes occurring as porphyritic crystals in some basalts, andesites and porphyrites, and known as basaltic horn blende are brown in thin slices and contain significant proportions of titanium. They are well known from the basalts of Bilin and Schima (Czechoslovakia). They are distinguished by a very low optical extinction angle on the plane of symmetry. Hornblende is a common mineral of many types of igneous rocks as in granite, syenite and especially diorite; also in the crystalline schists.
The alkali amphiboles are distinguished by their very compli cated composition and little is known of their constitution. They appear to form with hornblende a series of solid solutions which are represented by alkali amphiboles of intermediate composition. The alkali amphiboles may be subdivided broadly into two groups according to their composition, habit and mode of occur rence : an iron-poor light blue coloured group which are especially characteristic of the crystalline schists and an iron-rich dark blue coloured group which occur as prominent constituents of alkaline igneous rocks.
The first group is prominently represented by glaucophane and its alumina-rich relative gastaldite, while the second group is rep resented by the arfvedsonite-riebeckite series of amphiboles.
The arfvedsonite-riebeckite series occur in the alkaline igneous rocks such as nepheline syenite, tinguaite and phonolite. Rie beckite was first described from the granites of the island of Socotra. To be classed here is the hastingsite of the nepheline syenites of Ontario. Crossite, an alkali amphibole in the crystal line schists of Berkeley, California, is distinguished by possessing an optic axial plane normal to the plane of symmetry. Barke vikite, related to arfvedsonite is a brown pleochroic amphibole originally described from Barkevik (Norway).
The three minerals of the aenigmatite group, viz., aenigmatite, cossyrite and rhoenite are usually classed as triclinic amphiboles.
See also HORNBLENDE and TREMOLITE.
(C. E. T.)