analyses will serve to illustrate the character of the changes thus produced: Analyses of pyroxene rocks before and after alteration. Aa. The plagioclase-pyroxene rock of the Scourie dike. Ab. The derived hornblende schist. Analyses by Teall, loc cit. Ba. Pyroxene from the center of a crystal, Templeton, Canada. Bb. Intermediate portion of the same crystal. Bc. Hornblende forming the rim of the crystal. Analyses B by B. J. Harrington, Geol. Survey Canada, Rec. of Progress, 1877-78, p. 21 G. Ca. Diallage from a gabbro, Transvaal, South Africa. Cb. Uralite from alteration of the diallage. Analyses by P. Dahms, Neues Jahrb., Beil. Bd. 7, p. 99, 1891. That the change from pyroxene to uralite or amphibole is something more than a paramorphism these few analyses clearly show. In A there has been oxidation of ferrous to ferric iron, in B a loss of lime, and in C a loss of magnesia. In many cases uralitization is accompanied by a separation of magnetite," and the lime removed reappears as calcite. Epidote is also a common product during the process, which must vary with variations in the composition of the altering rock and of the individual pyroxene. Augite thus yields hornblende or actinolite; diopside may change into tremolite, and from the soda pyroxenes the aluminous glaucophane may be derived. The composition of the pyroxene is reflected in that of its derivative, but the augite-hornblende change is the most common. Between the original igneous rock and the secondary amphibolites, there are all possible intermediate gradations, from incipient change to complete transformation. GLAUCOPHANE SCHISTS. The glaucophane schists differ from the amphibolites in that they contain the soda amphibole instead of hornblende. H. S. Wash See G. Rose, Zeitschr. Deutsch, geol. Gesell., vol. 16, p. 6, 1864; E. Svedmark, Neues Jahrb., 1877, p. 99. See discussion of the change by C. H. Gordon, Am. Geologist, vol. 34, p. 40, 1904. ington divides these rocks into three classes, namely, epidote-glaucophane schist, mica-glaucophane schist, and quartz-glaucophane schist; but he also recognizes the fact that there are many transitional varieties. W. H. Melville, for example, has described a garnetglaucophane schist from Mount Diablo, California; and A. Wichmann an epidote-mica-glaucophane schist from Celebes. A zoisiteglaucophane schist from Sulphur Bank, California, is also mentioned by G. F. Becker. It consists chiefly of glaucophane and zoisite, but quartz, albite, sphene, and muscovite are also present. Another rock from Piedmont, containing glaucophane, garnet, hornblende, epidote, mica, and sphene, described by T. G. Bonney, is called a glaucophane eclogite. Genetically, the glaucophane rocks differ widely. Some of them are undoubtedly derived from mediosilicic or subsilicic igneous rocks; others from sedimentaries. In Greece, for example, according to R. Lepsius, some glaucophane schists represent gabbro, and others are metamorphosed Cretaceous shales. The epidote-glaucophane schist of Anglesey, Wales, described by J. F. Blake, was originally a diorite, and in this rock alterations of glaucophane to chlorite occur. In Piedmont, as described by S. Franchi," there are glaucophane rocks associated with amphibolite, both having been derived from diabase. In Japan, according to B. Koto, the metamorphosed material was formerly a diabase tuff, and the glaucophane was derived from diallage. By further alteration the glaucophane sometimes passes into crocidolite. And on Angel Island, in San Francisco Bay, California, a glaucophane schist studied by F. L. Ransome has been developed from a radiolarian chert, probably by contact metamorphism. In many cases the genesis of these rocks is obscure; but Washington suggests that the epidote-glaucophane schists represent originally gabbroid magmas, while the quartz-glaucophane schists are metamorphosed quartzites or quartzose shales. For convenience, differences of origin will be disregarded here and the analyses of this group of rocks are tabulated together, as follows: a Am. Jour. Sci., 4th ser., vol. 11. p. 35, 1901. This memoir is a very complete summary of our knowledge of these rocks. It contains many analyses, and abundant references to literature. See also K. Oebbeke, Zeitschr. Deutsch. geol. Gesell., vol. 38, p. 634, 1886; U. Grubenmann, Rosenbusch Festchrift," 1906; E. H. Nutter and W. B. Barber, Jour. Geol., vol. 10, p. 738, 1902; and J. P. Smith, Proc. Am. Phil. Soc., vol. 45, p. 183, 1906. The last two papers relate to the glaucophane rocks of California. Bull. Geol. Soc. America, vol. 2, p. 413, 1890. Neues Jahrb., 1893, pt. 2, p. 176. Mon. U. S. Geol. Survey, vol. 13, p. 104, 1888. Mineralog. Mag., vol. 7, p. 1. 1887. 1 Geologie von Attika, pp. 102, 133, Berlin, 1893. Geol. Mag., 1888, p. 125. A Bol. Com. geol. ital., vol. 26, p. 192, 1895. Jour. Coll. Sci. Japan, vol. 1, p. 85, 1886. Bull. Dept. Geology, Univ. California, vol. 1, p. 211. Analyses of amphibolites and glaucophane schists.a Analyses A and B by W. F. Hillebrand, Bull. A. Amphibolite dike, Palmer Center, Massachusetts. C. Amphibolite, Crystal Falls district, Michigan. Described by H. L. Smyth, Mon. U. S. Geol. Survey, vol. 36, p. 397, 1899. Analysis by H. N. Stokes. Probably derived from a diabase or basalt. Contains hornblende, plagioclase, biotite, and quartz, with a little rutile and magnetite. D. Epidote-glaucophane schist, Mount Diablo, California. Analysis by W. H. Melville, Bull. Geol. Soc. America, vol. 2, p. 413, 1890. Contains garnets. Possibly derived from shale. E. Garnet-glaucophane schist, Bandon, Oregon. Analyzed and described by H. S. Washington, Am. Jour. Sci., 4th ser., vol. 11, p. 35, 1901. Contains glaucophane, epidote or zoisite, garnet, and white mica. F. Zoisite-glaucophane schist, Sulphur Bank, California. Analysis by Melville. Described by G. F. Becker, Mon. U. S. Geol. Survey, vol. 13, p. 104, 1888. G. Mica-glaucophane schist, Island of Syra, Greece. Analyzed and described by Washington, loc. cit. H. Quartz-glaucophane schist, Fourmile Creek, Coos County, Oregon. Analyzed and described by Washington. Contains quartz, glaucophane, chlorite, muscovite, and garnet. A number of amphibolites from Massachusetts are described by B. K. Emerson in Mon. U. S. Geol. Survey, vol. 29, 1898. For analyses see also Bull. No. 228, pp. 34, 35, 1904. These amphibolites are regarded as derived from argillaceous limestones. L. Hezner (Min. pet. Mitth., vol. 22, p. 505, 1903) gives several analyses of amphibolites from the Tyrol. See also a table of analyses in Rosenbusch, Elemente der Gesteinslehre, 532. See also, on amphibolite, F. Becke, Min. pet. Mitth., vol. 4, p. 285, 1882; and J. Á. Ippen, Mitth. Naturwiss. Ver. Steiermark, 1892, p. 328. Ippen describes "normal amphibolite," and also zoisite-, pyroxene-, feldspar-, and garnet-amphibolite. SERICITIZATION. The conversion of feldspar into muscovite is one of the commonest processes of metamorphism, whether of igneous or of sedimentary rocks. In many instances the mica produced is the compact or fibrous variety known as sericite, which, in former times, was generally mistaken for talc. The so-called talcose schists of the earlier geologists have proyed in most cases to be not talcose, but sericitic. The identity of sericite with muscovite was finally established by H. Las See G. H. Williams, Bull. U. S. Geol. Survey No. 62, pp. 60-62, 1890, for historical details. 1 peyres a in 1880, and since then its occurrence has been repeatedly investigated. The alteration is most conspicuous in regions where the dynamic metamorphism has been most intense-high temperature, the chemical activity of water, and mechanical stress all working together to bring it about. Any feldspathic rock may undergo sericitization, but orthoclase rocks furnish the most typical examples. The derivation of sericitic schists and gneisses from granite, quartz porphyry, and diabase, and also from arkose and clay slate, has been repeatedly observed." Sericite is commonly derived from orthoclase or microcline, as suggested above, but may be generated from plagioclase feldspars also, the reactions in the two cases being different. In the formation of muscovite from orthoclase the necessary potassium is already present; but in order to produce muscovite from plagioclase a replacement of sodium by extraneous potassium is required. In either case the reaction which takes place may be represented by more than one equation, although it must be admitted that the formulation is purely hypothetical. Until the processes shall have been experimentally reproduced the equations will remain doubtful. First, orthoclase may be transformed to muscovite by the addition of colloidal alumina equivalent in composition to diaspore, thus: KAISI,O,+2A1O.OH=KH2Al3Si3O12. This reaction is very simple chemically, but geologically improbable. It requires the presence of solutions containing much alumina, and it is not easy to see whence they could be derived. It suggests, however, a possible relation between the formation of sericite and the alteration to bauxite, a possibility which deserves further investigation. A second, more probable, and even simpler reaction is the following: 3KAlSi ̧ ̧+H2O=KH,Al2Si ̧O12+K,SiO+5SiO2. In this case water alone, acting on orthoclase at a high temperature and under pressure, forms muscovite, free silica, and potassium silicate, the last compound being leached away. The liberated silica may be partly removed in solution, or it can recrystallize as quartz, a mineral which almost invariably accompanies sericite in metamorphic rocks. Furthermore, the analyses of sericite usually show a Zeitschr. Kryst. Min., vol. 4, p. 245, 1879. See J. G. Lehmann, Untersuchungen über die Entstehung der altkrystallinischen Schiefergesteine, Bonn, 1884; A. Wichmann, Verhandl. Natur. Ver. preuss. Rheinl. u. Westfalens, vol. 34. p. 1, 1877; A. von Groddeck, Neues Jahrb., Beil. Bd. 2, p. 72, 1883; C. Schmidt, idem, Beil. Bd. 4. p. 428, 1886, and C. Benedicks, Bull. Geol. Inst. Upsala, vol. 7, p. 278, 1904-5. In Jahrb. K. preuss. geol. Landesanstalt, 1885, pt. 1, Von Groddeck describes the derivation of sericite schists from clay slates. small excess of silica over that contained in normal muscovite. A similar reaction with albite should yield the soda mica paragonite. A modified form of the last reaction is in common use, which involves the introduction into the equation of carbonated water, as follows: 3KAISI,O,+H2O+CO„=KH¿Al ̧Si„O12+K,CO1+6SiO2. 3 8 In this case, however, the potassium carbonate would dissolve one molecule of the liberated silica, forming potassium silicate as before. The CO, would thus be set free again, ready to assist in further alterations of feldspar. Since carbonated waters, both of meteoric and of deep-seated origin, are very abundant, it is quite possible that this regenerative process is really in operation. If so, the reaction should be more vigorous than when water acts alone. The frequent association of calcite with sericite is an indication that carbonated solutions have helped to produce the change." If the alteration took place in presence of both albite and orthoclase, the potassium silicate would probably react upon the former mineral or upon its incipient decomposition products, so that muscovite only, without paragonite, would be formed. In the development of muscovite from plagioclase the presence of potassium-bearing solutions, which exchange alkalies with the sodium compounds, must be assumed. OTHER ALTERATIONS OF FELDSPAR. Apart from the phenomenon of sericitization, the plagioclase feldspars undergo a number of other metasomatic changes, whose records are preserved in the metamorphic rocks. Under the influence of carbonated waters the anorthite molecule may be decomposed, with the formation of calcite and the separation of silica. In this case the albite remains as a finely granular aggregate, the so-called "albite mosaic," which outwardly resembles quartz and with which quartz is commonly associated. When the lime of the anorthite is not completely removed, it goes to form other silicates, such as epidote, zoisite, or actinolite. The latter reactions are by far the most frequent. The alteration of plagioclase to zoisite is exceedingly common, but it is rarely complete. As a rule mixtures of zoisite and feldspar remain, which were once thought to represent a distinct mineral species and to which the name saussurite was given. The mechanism of the change is obscure, but it is probably a double decomposition between the albite and anorthite molecules, brought about by the intervention of water. The feldspars, however, vary in composition; the water may contain other reacting substances in solution, and so a Compare W. Lindgren, Trans. Am. Inst. Min. Eng., vol. 30, p. 608, 1900, in reference to the association with calcite. See K. A. Lossen, Jahrb. K. preuss, geol. Landesanstalt, 1884, pp. 525–530. See also G. H. Williams, Bull. U. S. Geol. Survey No. 62, p. 60, 1890. |