AMPHIBOLITE, in petrology the name given to a grey, green or black, typically non-schistose rock consisting mainly of amphibole and a felspar of the plagioclase group, the use of the term being restricted, however, to rocks of metamorphic origin. The term may be contrasted with hornblende schist, which is of identical composition but possesses a well-defined schistose struc ture as compared with the granoblastic unfoliated structure of the amphibolite. As is the case with most petrological terms the exact connotation is not very strictly defined, and some authors reserve the term hornblende schist for schistose rocks built up essentially of amphibole. As accessory constituents may be noted a red lime-magnesia-iron garnet, epidote, zoisite, green pyroxene, scapolite, biotite, etc.
The commonest amphibole is the aluminous monoclinic horn blende. Other varieties, however, occur, such as tremolite, actino lite, the soda amphibole glaucophane and, more rarely still, cross ite. The magnesia-iron amphiboles of the cummingtonite-gruner ite series, and the rhombic amphiboles anthophyllite and gedrite form essential constituents of certain amphibolites. The usual plagioclase felspar is an andesine or labradorite ; in this zoning and lamellar twinning are often less marked than in the f elspars of igneous rocks.
The amphibolites are typical rocks of the metamorphic group and as such attain a large development in all regions of crystalline schists and gneisses as in the old preCambrian shields and in met amorphic areas of later date. They occur principally in stocks or bosses representing basic igneous intrusions, or as long, narrow strips intercolated among outcrops of other metamorphic rocks. Genetically they fall into two groups, the ortho-amphibolites, which are transmuted igneous rocks, and the para-amphibolites, the metamorphic representatives of sedimentary rocks. The for mer are much the more common, and of these the greater num ber are derived without significant change in bulk composition from gabbros and dolerites. Igneous rocks containing much augite pass under the conditions prevailing in regional metamorphism where shearing stress is dominant into rocks of this kind, the pyroxene being converted to amphibole often with separation of iron ores and sphene. From more alkaline basic rocks the soda amphibole, glaucophane, is produced.
Plagioclase felspar is recrystallized with or without separation of some of the anorthite molecule as epidote or zoisite according to the grade of metamorphism imposed. The rock may thus be completely transformed into a granoblastic aggregate of new f ormed minerals with entire loss of all igneous textures. Fre quently, however, this transformation is only partial, and exami nation of a large number of specimens reveals a series of interme diate or transitional stages, proving conclusively the nature of the processes at work.
The sedimentary amphibolites or para-amphibolites, less com mon than those above described, are frequent in some districts, such as the northern Alps, southern highlands of Scotland, Green mountains, U.S.A. Many of them have been ash-beds, and their conversion into hornblende-schists follows exactly similar stages to those exemplified by basic crystalline igneous rocks. Others have been greywackes of varied composition with epidote, chlo rite, felspar, quartz, iron oxides, etc., and may have been mixed with volcanic materials, or may be partly derived from the disin tegration of basic rocks. When they are most metamorphosed they are often very hard to distinguish from igneous hornblende schists ; yet they rarely fail to reveal signs of bedding, pebbly structure, sedimentary banding and gradual transition into un doubtedly sedimentary types of gneiss and schist.
Impure dolomitic limestones yield on metamorphism para amphibolites strikingly similar to some ortho-amphibolites. By mutual reaction of chlorite, kaolin, calcite and dolomite amphi boles and plagioclase felspars are produced. The original nature of these highly altered rocks, however, is frequently displayed in the rapid variation in mineral composition followed across the strike, corresponding to initial inhomogeneity of the original sedi ment. Amphibolites of this type are common in the Haliburton Bancroft area of Ontario, where they occur, together with ortho amphibolites, as enclosures in the Laurentian gneiss.
(C. E. T.)