Petrological Abstracts and Reviews

Albert Johannsen
1918 The Journal of geology  
In this volume the author attempts to answer two important questions in chemical petrography. First, what are the laws of the chemical composition of igneous rocks, and what are the characteristics separating the rocks of the alkali series from those of the alkali-lime? Second, what are the most important chemical differences between sediments and igneous rocks, and how may these be made of value in determining the origin of crystalline rocks ? To answer these questions, use is made of nearly
more » ... irteen hundred chemical analyses. The relationships between the oxides which vary most in their proportions in sediments and igneous rocks and which are also of value in comparing the igneous rocks, have been determined. Molecular proportions are used and recomputed to a constant sum so that they may be diagrammatically compared. Unimportant and accessory constituents are omitted, but TiO2 and ZrO2 are added to SiO2, BaO and SrO to CaO, and MnO to iron, which is calculated as FeO. The four relationships computed are: i. SiO2:Al03: (Fe, Mg, Ca)O= SA1F proportions. In this series the alkalies are entirely omitted. In sedimentary rocks this relationship leads to a grouping in three classes; siliceous, aluminous, and calcareous. 2. Al203:CaO: (Na,K),O=AlCAlk proportions. This shows the more important differences between sediments and igneous rocks, and, in combination with the preceding, the characteristic differences between the alkali and alkali-lime series. 3. Na20:KO2= NK proportions. Recalculated to 1o. 4. MgO: CaO = MC proportions. Recalculated to 1o. The last two are of importance in separating igneous rocks from sediments. The first two may be represented graphically in a triangular diagram after recalculation to 30. The results are plotted to the nearest 0.5. A 272 This content downloaded from on September 18, 2016 00:32:43 AM All use subject to University of Chicago Press Terms and Conditions ( PETROLOGICAL ABSTRACTS AND REVIEWS recalculation to 1oo and representation to the nearest whole number seems simpler to the reviewer, especially if the calculation is performed on a slide-rule. Pp. 68 to 161 are devoted to nearly 1,300 analyses recomputed to these four values; the chemical values used are taken from Osann's former tables and are referred to by number. In Table I the rocks are arranged according to the decreasing S, increasing Al, and decreasing F values; in Table II according to the AlCAlk values. Table III is arranged under the ordinary rock-names, the SA1F, etc. values being given as well as references to the literature. Of all the analyses computed, only two coincided in all four relationships, namely, the quartz-monzonite from Elkhorn, Montana, and an average of four analyses of the Butte "granite." Since the two rocks are from the same batholith, the similarity is not surprising. To follow the discussions of the various relationships, it is necessary to look at the accompanying diagrams. No attempt, therefore, will be made to summarize them here, and the reader is referred to the original paper. While the present work deals only with the igneous rocks, a number of examples of sedimentaries and crystalline schists are given for comparison. A second part, dealing with sediments and schists is to follow. This work represents an enormous amount of patient labor. It should be of extremely great value for the visualization of chemical differences in rocks. OSANN, A. "Uber topische Gesteinsparameter," Sitzb. Heidelberger Akad. Wiss. Math.-naturw. Kl., 1.914, A 26, pp. 15, pls. 3, fig. I. Rocks of a petrographic province, or from a rock-mass showing zonal or other differentiation, or from any igneous body and its satellites, when plotted in a triangular diagram after Osann's well-known system, show clearly their mutual relationships. If it is desired to show the general relationships of any igneous to the average igneous rock, it is only necessary to indicate the latter by a point properly placed. In this paper the mean of Clarke's and Washington's average rocks is taken and is recalculated in the Osann system. This gives a rock not far from the pyroxene-amphibole-biotite-diorite from Electric Peak. The latter, however, has more dioritic characters, the former more monzonitic. To show more clearly the variations of other rocks from this mean, it is shifted to the center of the triangle and the co-ordinates 273 This content downloaded from on September 18, 2016 00:32:43 AM All use subject to University of Chicago Press Terms and Conditions ( PETROLOGICAL ABSTRACTS AND REVIEWS RINNE, F. "Beitrag zur optischen Kenntnis der kolloidalen Kieselsaure," Neues Jahrb. Min., Geol., u. Pal., B.B. XXXIX (1914), 388-414, figs. 12. Describes apparatus and results obtained by the examination at different temperatures, between -6000 and +10000, and by light of different wave lengths, of various amorphous silicates, such as quartzglass, hyalite, precious opal, moldavite, obsidian, and marekanite. It was found that these substances fell into two groups, water-free or water-poor, and water-rich. In the former the refractive indices progressively increased with temperature changes, while in the latter they increased to the neighborhood of o° and then decreased. RINNE, F. "Die Kristallwinkelveranderung verwandter Stoffe beim Wechsel der Temperatur. I," Centralbl. Min., Geol., u. Pal., 1914, 705-18, figs. 9. With the apparatus described in the preceding paper, the author found that the angle of the rhombohedron (ioli) in calcite, dolomite, siderite, and rhodochrosite increased with increasing temperature. Above o° the curve is a straight line, below o° it is slightly curved. The plagioclase feldspars show a decrease in the angle ooI-oio with increasing temperature. Albite shows the greatest change, anorthite the least. The curves are very flat at low temperatures and rapidly drop at high temperatures. SCHMIDT, EDUARD. "Die Winkel der kristallographischen Achsen der Plagioklase," Chemie der Erde, I (1915), 351-406, figs. 13, bibliography. A study of the plagioclase feldspars, unusually valuable since the material was analyzed. There are new determinations of the cleavage angle (ooi): (oio), which show that this angle is a linear function of the An. content. The specific gravity determinations, with one exception, agree very well with the determinations made by Day and Allen on artificial feldspars. The value for the labradorite from Labrador is given as 2 .689+0.003, and its composition as An. between 49 and 50; the artificial feldspar AbAn, gives a value, according to Day and Allen, of 2.679. The material from Labrador, however, was zonal, and the An. percentage as computed from the silica, lime, alkalies, etc., varied between 44.8 and 55.8. It is possible, therefore, that the anorthite 279 This content downloaded from on September 18, 2016 00:32:43 AM All use subject to University of Chicago Press Terms and Conditions ( PETROLOGICAL ABSTRACTS AND REVIEWS percentage should have been taken higher. There are also new values for the angles between crystallographic a and c (/f) and between a.and b (y). The Purcell sills represent, according to the author, intrusions from a single intercrustal reservoir, which he apparently believes contained the magma already differentiated according to density, the relatively acid portion collected in irregularities and projections of the roof of the chamber and grading downward into more basic materials. Crustal movements produced fissures which tapped the reservoir at various levels, so that acid and basic materials would rise through separate fissures and spread between the overlying strata as sills. The sills themselves are simple or composite. The former solidified in the usual manner of intrusives; the composite sills differentiated in place, some of them having basic upper and lower contacts and an inner portion which is more acid in the upper part and more basic in the lower. The composition of the sills may be slightly modified by assimilated material derived from included fragments or from the inclosing rock.
doi:10.1086/622589 fatcat:jm7mznm6hrfrhcxilkbdd5ntz4