Etna Part 7

The t.i.taniferous iron of Etna is found disseminated through the ma.s.s of the lavas, and is plainly distinguished when a thin section is examined under the microscope. It is sometimes met with in ma.s.ses. A specimen from Etna, a.n.a.lysed by Von Waltershausen, was found to contain:--

t.i.tanic Acid 1114 Sesquioxide of Iron 5886 Protoxide of Iron 3000 ------ 10000

The basalts of the Isole de'Ciclopi enclose beautiful transparent crystals of a.n.a.lcime, the _zeolite dure_ of Dolomieu. The word is derived from ~a.n.a.lkis~ weak, in allusion to the weak electric power which the mineral acquires when heated or rubbed. Dana prefers the term _a.n.a.lcite_. Specimens from the Cyclops Islands have been a.n.a.lysed by Von Waltershausen and Rammelsberg, with the following results:--

I. II. III.

Silica 5372 5522 5434 Alumina 2403 2314 2361 Lime 123 25 21 Soda (Na{2}O) 792 1219 1295 Potash (K{2}O) 446 152 66 Water 850 768 811 Magnesia 05 -- -- Sesquioxide of Iron -- -- 12 ----- ------ ------ 9991 10000 10000

The minerals of Etna are not nearly as numerous as those of Vesuvius. It has been remarked that no area of equal size on the face of the globe furnishes so many different species of minerals as Vesuvius and its immediate neighbourhood. Out of the 380 species of simple minerals enumerated by Hauy, no less than 82 had been found on and around Vesuvius, as long ago as 1828, and many have been since found.

Of other common products of Etna, there are sulphur in various forms, sulphurous acid gas, ammonia salts, hydrochloric acid gas, and steam. A curious white ma.s.s, which we found near the summit, proved to be the result of the decomposition of lava by hot acid vapours. In the different lavas, the crystals of labradorite, and of olivine, vary in size considerably. Magnetic oxide of iron is very visible in thin slices of the lavas when placed under the microscope; and iron appears to be a constant const.i.tuent in nearly all the products of the mountain.

Within the last few months Prof. Silvestri has detected a mineral oil in the cavities of a prehistoric doleritic lava found near Paterno.[21] The lava is in close contiguity to the clay deposits of a mud volcano, and when examined under the microscope is seen to consist mainly of augite, together with olivine and transparent crystals of labradorite. It contains numerous cavities coated with arragonite, and filled with a mineral oil which const.i.tutes about one per cent of the whole weight of the lava. It was taken from the lava at a temperature of 24 C., (752 F.), and solidified at 17 C. (626 F.) to a yellowish green ma.s.s, which on a.n.a.lysis gave the following percentage composition:--

Liquid hydrocarbons boiling at 79 C. = 1797 Hydrocarbons solidifying below 0 C., boiling } between 280 and 400 C. } = 3195 Paraffine melting between 52 and 57 C. = 4279 Asphalt containing 12 per cent of ash = 290 Sulphur = 432 ------ 9993

[21] "Atti Accademia Gioenia," serie iii., vol. xii.

Prof. Silvestri has recently made some interesting determinations of the specific gravity and chemical composition of the different products of Etna. They are given in full in his work ent.i.tled, "_I Fenomeni Vulcanici presentati dall'Etna, nel 1863, 1864, 1865, 1866_," which was published in Catania in 1867. The following table gives the specific gravity of various ancient and modern forms of lava, ashes, etc. of Etna:--

_Sp. Gr._

Ashes ejected in 1865 2644 Sand " " " 2715 Scoriae " " " 2633 Compact lava " " 2771 Scoriae ejected in 1669 2622 Compact lava " " 2697 Lapilli ejected in 1444 2420 Compact lava ejected in prehistoric times 2854

A very decided change in the specific gravity was found to take place after fusion. This can only be accounted for on the supposition that a chemical change is effected during the fusion:--

_Sp. Gr._ _Sp. Gr._ _before fusion._ _after fusion._

Pyroxene of Etna 3453 2148 Felspar " " 2925 1361 Olivine " " 3410 2290 Lava of 1865 2771 1972 Ancient basaltic lava from } 2854 2000 the Scogli de'Ciclopi } Ancient basaltic lava from } 2795 1947 Aci Reale }

It will be seen from the following a.n.a.lyses that the sand, ashes, scoriae, and compact lava have virtually the same composition--indeed they consist of the same substance in different states of aggregation.

_Ashes._ _Sand._ _Scoriae._ _Compact lava._

Silica 5000 4980 5000 4995 Alumina 1908 1820 1900 1875 Protoxide of iron 1216 1242 1170 1121 Protoxide of Manganese 40 45 50 49 Lime 998 1100 1028 1110 Magnesia 412 400 420 405 Potash 60 49 69 70 Soda 372 360 340 371 Water 36 29 33 23 Phosphoric acid } t.i.tanic acid } traces traces traces traces Vanadic acid } Sesquioxide of iron } ------ ------ ------ ------ 10042 10025 10010 10019

With these we may compare the composition of the lava which issued from Monti Rossi in 1669, and was a.n.a.lysed by Lowe, and of an ancient lava of Etna ejected during an unknown eruption, and a.n.a.lysed by Hesser.

_Ancient lava._ _Lava of 1669._

Silica 4963 4883 Alumina 2247 1615 Protoxide of Iron 1080 1632 Protoxide of Manganese 63 54 Lime 905 931 Magnesia 268 458 Soda 307 345 Potash 98 77 ----- ----- 9931 9995

The sublimations from the fumaroles are chiefly chloride of ammonium, perchloride of iron, and sulphur. An a.n.a.lysis of the gases of the fumaroles of 1865 gave the following results:--

Carbonic acid 505 Hydrosulphuric acid 119 Oxygen 71 Nitrogen 305 ----- 1000

An account of microscopic a.n.a.lysis of some of the lavas of Etna, for which I am indebted to Mr. Frank Rutley, will be found appended to this chapter. He considers that they are Plagioclase-basalts, and occasionally Olivine-basalts; and that they consist of Plagioclase, Augite, Olivine, Magnet.i.te, t.i.taniferous iron, and a residuum of gla.s.s.

Near the summit of the great crater I found a ma.s.s of perfectly white, vesicular, and very friable substance, somewhat pumiceous in appearance.

It proved to be a decomposed lava, and was found elsewhere on the sides of the crater. Mr. Rutley examined a section of it, and reports: "Under the microscope a tolerably thin section shows the outlines of felspar crystals, lying in a hazy milk-white semi-opaque granular matrix. The felspar crystals are lighter and more translucent than the matrix, but are of much the same character, having a granulated or flocculent appearance, somewhat like that of the decomposed felspars in diabase.

There are numerous roundish cavities in the section which may once have contained olivine, or some other mineral, or they may be merely vesicles."

A qualitative a.n.a.lysis of this substance, made by Mr. H. M. Elder, has proved that it contains a large quant.i.ty of Silica (about 70 per cent.), and smaller proportions of Alumina, Iron, Magnesium, Calcium, and Potash; together with very small amounts of Sulphuric Acid and a trace of Ammonia. Lithium is absent, and Sodium is only present in very minute quant.i.ty. Water is present to the extent of nearly 20 per cent.

During the eruption of Etna in 1869 Von Waltershausen noticed on some of the lava blocks which were still hot and smoking, silver-coloured particles, which rapidly underwent change. An insufficient quant.i.ty for a.n.a.lysis was collected, but during the eruption of 1874, Silvestri found a quant.i.ty of the substance and a.n.a.lysed it. (_Poggendorff's Annalen_, CLVII. 165, 1876.) It possesses a specific gravity of 3147, and shows a metallic l.u.s.tre similar to that of steel. On a.n.a.lysis it was found to consist of:--

Iron 9086 Nitrogen 914 ------ 10000

which corresponds with the formula Fe{5}N{2},--a formula a.s.signed by Fremy to Nitride of iron. It has been named _Siderazote_. This new mineral species appears to be formed by the action of hydrochloric acid, and of ammonia on red-hot lava containing a large percentage of iron. It was formed artificially by exposing fragments of lava alternately to the action of hydrochloric acid and ammonia in a red-hot tube. At a high temperature Siderazote undergoes decomposition, nitrogen being evolved.

In contact with steam at a red heat it forms magnet.i.te and ammonia.

_The Mineral Const.i.tution 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 const.i.tution. 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 const.i.tuents, yet in some of them certain minerals fluctuate in quant.i.ty, 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 spa.r.s.ely-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 ma.s.s 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, a.n.a.lcime, 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 gla.s.s distinctly in a microscopic section of the lava of Salto di Pulich.e.l.lo. In the other sections which I have examined there appears to be a small quant.i.ty of interst.i.tial gla.s.s, but it is so finely disseminated between the microliths of felspar and granules of olivine, augite and magnet.i.te, which const.i.tute the ground-ma.s.s 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.

[Ill.u.s.tration: Sections of Etna Lavas seen under the Microscope]

Plagioclastic felspars are unquestionably the dominant const.i.tuents 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 ent.i.tle the rock to the appellation Trachy-dolerite.

Augite and olivine are generally present in the Etna lavas, especially the latter mineral.

Magnet.i.te appears to occur in all of them. t.i.taniferous 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 const.i.tuent 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, magnet.i.te, and, in some cases, sanidine--possibly t.i.taniferous iron--and in some, if not in all, a slight residuum of gla.s.s. 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 lamellae 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 gla.s.s, containing fine dark granular matter--probably magnet.i.te, which often renders them opaque,--or of matter similar to that which const.i.tutes the groundma.s.s of the surrounding rock. These stone and gla.s.s 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 lamellae. Zirkel has noted the plentiful occurrence of these gla.s.s inclosures in the felspar crystals and fragments of crystals which partly const.i.tute the volcanic sands of Etna, in which he has also detected the presence of numerous isolated particles of brownish gla.s.s.[23] The felspar microliths, which const.i.tute so large a proportion of the ground-ma.s.s 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 lamellae.

[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 gla.s.s, probably containing magnet.i.te dust and matter similar to that of the groundma.s.s of the rock which consists of felspar microliths, granules of olivine, and augite crystals, grains of magnet.i.te, and apparently a little interst.i.tial gla.s.s. 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. Groundma.s.s 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.

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.