|
Titanic Acid Sesquioxide of Iron Protoxide of Iron |
11·14 58·86 30·00 100·00 |
The basalts of the Isole de' Ciclopi enclose beautiful transparent crystals of Analcime, the zeolite dure of Dolomieu. The word is derived from ανα'λκις weak, in allusion to the weak electric power which the mineral acquires when heated or rubbed. Dana prefers the term analcite. Specimens from the Cyclops Islands have been analysed by Von Waltershausen and Rammelsberg, with the following results:—
|
Silica Alumina Lime Soda (Na2O) Potash (K2O) Water Magnesia Sesquioxide of Iron |
I. 53·72 24·03 1·23 7·92 4·46 8·50 ·05 — 99·91 |
II. 55·22 23·14 ·25 12·19 1·52 7·68 — — 100·00 |
III. 54·34 23·61 ·21 12·95 ·66 8·11 — ·12 100·00 |
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 mass, 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 constituent 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 constitutes about one per cent of the whole weight of the lava. It was taken from the lava at a temperature of 24° C., (75·2° F.), and solidified at 17° C. (62·6° F.) to a yellowish green mass, which on analysis gave the following percentage composition:—
| Liquid hydrocarbons boiling at 79° C. | = 17·97 |
| Hydrocarbons solidifying below 0° C., boiling between 280° and 400° C. | = 31·95 |
| Paraffine melting between 52° and 57° C. | = 42·79 |
| Asphalt containing 12 per cent of ash | = 2·90 |
| Sulphur | = 4·32 |
| 99·93 |
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 entitled, "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 Sand ,, ,, ,, Scoriæ ,, ,, ,, Compact lava ,, ,, Scoriæ ejected in 1669 Compact lava ,, ,, Lapilli ejected in 1444 Compact lava ejected in prehistoric times |
2·644 2·715 2·633 2·771 2·622 2·697 2·420 2·854 |
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:—