The Mineral Constitution and Microscopic Characters of Some of the Lavas of Etna
By Frank Rutley, f.r.g.s., of H.M. Geological Survey.
A cursory examination of the series of specimens collected by Mr. Rodwell, seemed to show that all the lavas of Etna, irrespective of their differences in age, exhibit a remarkable similarity in mineralogical constitution. Occasionally, however, there appears to have been a little difference in their respective viscidity at the time of the eruption, the crystals in some of them lying in all directions, while in others there appears to be a more or less definite arrangement of the felspar crystals, as seen in the lava of a.d. 1689.
Although the specimens which I have examined microscopically do not appear to differ in the nature of their constituents, yet in some of them certain minerals fluctuate in quantity, some containing a comparatively large amount of olivine and well-developed crystals of augite, while, in others, these minerals, although one or other is always present, are but poorly represented by minute and sparsely-disseminated grains. It seems probable that all the Etna lavas contain traces of a vitreous residuum, since, when sections are examined under the microscope, a more or less general darkness pervades their ground mass as soon as the Nicols are crossed, and this general darkness does not appear to be dissipated during the horizontal revolution of the sections themselves. The translucent minerals in these sections are all doubly refracting, and as I have not been able to detect the presence of hauyne, noseau, sodalite, analcime, or any other cubic mineral in them, the natural inference is that the obscurity between crossed Nicols is due to amorphous matter. I have only been able to ascertain the presence of glass distinctly in a microscopic section of the lava of Salto di Pulichello. In the other sections which I have examined there appears to be a small quantity of interstitial glass, but it is so finely disseminated between the microliths of felspar and granules of olivine, augite and magnetite, which constitute the ground-mass of these rocks, that it is most difficult to determine the single refraction of such minute specks during revolution between crossed Nicols, and I therefore merely express a belief, which, in some instances, I cannot demonstrate with any certainty.
Plagioclastic felspars are unquestionably the dominant constituents of these lavas. Lyell, in his “Principles of Geology,” (9th Edition, p. 411), states that the felspar is Labradorite. He does not, however, give the grounds for this conclusion, and, as microscopic examination alone merely indicates the crystalline system and not the species of felspar, it is unsafe to speculate upon this point in the absence of chemical investigation. In some of these lavas Sanidine is also present, but it is always subordinate to the plagioclase, and does not, as a rule, appear to play a part sufficiently prominent to entitle the rock to the appellation Trachy-dolerite.
Augite and olivine are generally present in the Etna lavas, especially the latter mineral.
Magnetite appears to occur in all of them. Titaniferous iron may also be represented, but I have failed to detect any well-defined crystals, or any traces of the characteristic white decomposition product which would justify me in citing the presence of this mineral, although it is stated by Lyell to occur in these rocks.
The constituent minerals of the Etna lavas now to be described, namely, those of b.c. 396 and a.d. 1535, 1603 and 1689, are:—
Plagioclase, augite, olivine, magnetite, and, in some cases, sanidine—possibly titaniferous iron—and in some, if not in all, a slight residuum of glass. These lavas must therefore be regarded as plagioclase-basalts, or occasionally as olivine-basalts. The plagioclase crystals vary greatly in size, some being mere microliths while others are over the eighth of an inch in length. They show the characteristic twin lamellation by polarized light, but the lamellæ are often very irregular as regards their boundaries. The sections of the crystals themselves are also frequently bounded by irregular outlines, but they often show internally delicate zonal markings, as indicated in Fig. 1,22 which correspond with the outlines of perfectly developed crystals. The inclosures in the larger plagioclastic felspars consist for the most part either of brownish glass, containing fine dark granular matter—probably magnetite, which often renders them opaque,—or of matter similar to that which constitutes the groundmass of the surrounding rock. These stone and glass cavities are very numerous and most irregular in outline, as shown in Figs. 1 and 2. They appear, however, to be elongated generally in the direction of the planes of composition of the twin lamellæ. Zirkel has noted the plentiful occurrence of these glass inclosures in the felspar crystals and fragments of crystals which partly constitute the volcanic sands of Etna, in which he has also detected the presence of numerous isolated particles of brownish glass.23 The felspar microliths, which constitute so large a proportion of the ground-mass in the Etna lavas, are in most instances probably triclinic. Monoclinic felspar does, however, occur in some of these rocks; but the difficulty of ascertaining the precise character of microliths renders it unsafe to speculate on the amount of sanidine which may be present. Some crystals, such as that shown in the centre of Fig. 2, appear at first sight to be sanidine, twinned on the Carlsbad type, but closer inspection often demonstrates the presence of other and very delicate twin lamellæ.
The augite in these lavas sometimes occurs in well-formed crystals of a green or brown colour, and often shows the characteristic cleavage very well, especially in the augite crystals of the lava of the Boccarelle del Fuoco, erupted in 1535. A small crystal of green augite is represented at the bottom of Fig. 1. Augite, however, appears to be more plentiful in the rocks in the form of small roundish grains.
Olivine is of very common occurrence in the Etna lavas, mostly in round or irregularly shaped grains, but also in crystals which usually exhibit rounded angles.
A specimen of lava from Salto di Pulichello, erupted in 1603, gave well-developed examples of the presence of olivine, and also of plagioclase. The ground mass was found to consist of felspar microliths, and grains of olivine, augite, and magnetite, with some interstitial glass.
Magnetite is present in all of the lavas here described. It occurs both in octahedral crystals and in the form of irregular grains and fine dust. To the presence of this substance much of the opacity of thin sections of the Etna lavas is due.
Titaniferous iron may also be present. One small crystal in the lava of 1535 appeared to show a somewhat characteristic form, but although much of the black opaque matter has undergone decomposition, I have failed to detect any of the white or greyish alteration product which characterises titaniferous iron, and in the absence of this, of definite crystalline form, and of chemical analysis, it seems better to speak of this mineral with reserve, although titanium is very probably present, since much magnetite is known to be titaniferous.
The vitreous matter which occurs in these lavas is principally present in the form of inclosures in the felspar, and, sometimes, the augite and olivine crystals previously described. Its occurrence in the groundmass of these rocks has also been alluded to. In this interstitial condition its amount is usually very small—a fact already pointed out by Zirkel.
I have unfortunately had no opportunity of examining the volcanic sands and ashes of Etna, but Zirkel’s description of them seems to indicate their close mineralogical relation to lavas erupted in this district, with one exception, as pointed out by Rosenbusch,24 namely, that he makes no mention of the occurrence of olivine in these ejectamenta.
Reference to the Figures 1 and 2 will suffice to show how close a relationship in mineral constitution exists between these two lavas, separated in the dates of their eruption by an interval of over two thousand years.
New Maps of Etna.—After these pages had received their final revision in type, I met with two new maps of Etna in the Paris Exhibition. The literature of our subject will obviously be incomplete without some notice of them, although this belongs properly to the first chapter rather than to the last. The one is a map in relief constructed by Captain Francesco Pistoja for the Istituto Topografico Militare of Florence. The vertical scale is 1/25,000 and the horizontal is 1/50,000. The surface is coloured geologically: the lavas erupted during each century being differently coloured, while the course of each stream is traced. This map, although by no means free from errors, is a vast improvement on the relief map of M. Elie de Beaumont. One defect, which might be easily remedied, is due to the fact that the lavas of three consecutive centuries are coloured so much alike, that it is almost impossible to distinguish them. The minor cones are well shown, the Val del Bove fairly well, and the map is altogether a valuable addition to our knowledge of the mountain.
The other map is a Carta Agronomica dell’ Etna, showing the surface cultivation. Different colours denote different plants, pistachio nuts, vines, olives, chestnuts, etc. It is beautifully drawn and coloured by hand, and is the work of Signor L. Ardini, of Catania.
Footnotes
22 The figures in this plate are magnified 35 diameters. Fig. 1. Lava of b.c. 396. The upper half of the drawing is occupied by a crystal of plagioclastic felspar showing twin lamellation and faint zonal markings, and with numerous irregular dark-brown inclosures of glass, probably containing magnetite dust and matter similar to that of the groundmass of the rock which consists of felspar microliths, granules of olivine, and augite crystals, grains of magnetite, and apparently a little interstitial glass. A crystal of augite is shown near the bottom of the drawing.
Fig. 2. Lava of a.d. 1689. On the right hand side part of a plagioclase crystal with inclosures similar to that in the preceding figure. In the centre a small crystal of plagioclase. Groundmass similar to that of Fig. 1, but showing a somewhat definite arrangement of the small felspar crystals, indicative of fluxion.
23 “Mikroskopische Beschaffenheit der Mineralien und Gesteine.” Leipzig, 1873; p. 480.
24 “Mikroskopische Physiographie der Massigen Gesteine. Stuttgart, 1877; p. 547.