The Student's Elements of Geology Part 64

Tremolite. St. Gothard (Rammelsberg)

58.55 .... .... .... 13.90 26.63 .... .... FW 0.34 2.93.

Actinolite. Arendal, in granite (Rammelsberg).

56.77 0.97 .... 5.88 13.56 21.48 .... .... W 2.20 3.02.

Hornblende. Faymont, in diorite (Deville).

41.99 11.66 .... 22.22 9.55 12.59 .... 1.02 W 1.47 3.20.

Hornblende Etna, in volcanic (Waltershausen).

40.91 13.68 .... 17.49 13.44 13.19 .... .... W 0.85 3.01.

Uralite. Ural (Rammelsberg)

50.75 5.65 .... 17.27 11.59 12.28 .... .... W 1.80 3.14.

Augite. Bohemia, in dolerite (Rammelsberg).

51.12 3.38 0.95 8.08 23.54 12.82 .... .... .... 3.35.

Augite. Vesuvius, in lava of 1858 (Rammelsberg).

49.61 4.42 .... 9.08 22.83 14.22 .... .... .... 3.25.

Diallage. Harz, in Gabbro (Rammelsberg).

52.00 3.10 .... 9.36 16.29 18.51 .... .... W 1.10 3.23.

Hypersthene. Labrador, in Labrador-Rock (Damour).

51.36 0.37 .... 22.59 3.09 21.31 .... .... .... 3.39.

THE OLIVINE GROUP.

1 2 3 4 5 6 7 8 9 10

Bronzite. Greenland (V. Kobell).

58.00 1.33 11.14 .... .... 29.66 .... .... .... 3.20.

Olivine. Carlsbad, in basalt (Rammelsberg).

39.34 .... .... 14.85 .... 45.81 .... .... .... 3.40.

Olivine. Mount Somma, in volcanic (Walmstedt).

40.08 0.18 .... 15.74 .... 44.22 .... .... .... 3.33.

The minerals which form the chief const.i.tuents of these igneous rocks are few in number. Next to quartz, which is nearly pure silica or silicic acid, the most important are those silicates commonly cla.s.sed under the several heads of feldspar, mica, hornblende or augite, and olivine. In Table 28.1, in drawing up which I have received the able a.s.sistance of Mr. David Forbes, the chemical a.n.a.lysis of these minerals and their varieties is shown, and he has added the specific gravity of the different mineral species, the geological application of which in determining the rocks formed by these minerals will be explained in the sequel.

From Table 28.1 it will be observed that many minerals are omitted which, even if they are of common occurrence, are more to be regarded as accessory than as essential components of the rocks in which they are found. (For a.n.a.lyses of these minerals see the Mineralogies of Dana and Bristow.) Such are, for example, Garnet, Epidote, Tourmaline, Idocrase, Andalusite, Scapolite, the various Zeolites, and several other silicates of somewhat rarer occurrence. Magnet.i.te, t.i.tanoferrite, and Iron-pyrites also occur as normal const.i.tuents of various igneous rocks, although in very small amount, as also Apat.i.te, or phosphate of lime. The other salts of lime, including its carbonate or calcite, although often met with, are invariably products of secondary chemical action.

The Zeolites, above mentioned, so named from the manner in which they froth up under the blow-pipe and melt into a gla.s.s, differ in their chemical composition from all the other mineral const.i.tuents of volcanic rocks, since they are hydrated silicates containing from 10 to 25 per cent of water. They abound in some trappean rocks and ancient lavas, where they fill up vesicular cavities and interstices in the substance of the rocks, but are rarely found in any quant.i.ty in recent lavas; in most cases they are to be regarded as secondary products formed by the action of water on the other const.i.tuents of the rocks. Among them the species a.n.a.lcime, Stilbite, Natrolite, and Chabazite may be mentioned as of most common occurrence.

QUARTZ GROUP.

The microscope has shown that pure quartz is oftener present in lavas than was formerly supposed. It had been argued that the quartz in granite having a specific gravity of 2.6, was not of purely igneous origin, because the silica resulting from fusion in the laboratory has only a specific gravity of 2.3. But Mr. David Forbes has ascertained that the free quartz in trachytes, which are known to have flowed as lava, has the same specific gravity as the ordinary quartz of granite; and the recent researches of Von Rath and others prove that the mineral Tridymite, which is crystallised silica of specific gravity 2.3 (see Table 28.1), is of common occurrence in the volcanic rocks of Mexico, Auvergne, the Rhine, and elsewhere, although hitherto entirely overlooked.

FELDSPAR GROUP.

In the Feldspar group (Table 28.1) the five mineral species most commonly met with as rock const.i.tuents are: 1. Orthoclase, often called common or potash- feldspar. 2. Albite, or soda-feldspar, a mineral which plays a more subordinate part than was formerly supposed, this name having been given to much which has since been proved to be Oligoclase. 3. Oligoclase, or soda-lime feldspar, in which soda is present in much larger proportion than lime, and of which mineral andesite are andesine, is considered to be a variety. 4. Labradorite, or lime- soda-feldspar, in which the proportions of lime and soda are the reverse to what they are in Oligoclase. 5. Anorthite or lime-feldspar. The two latter feldspars are rarely if ever found to enter into the composition of rocks containing quartz.

In employing such terms as potash-feldspar, etc., it must, however, always be borne in mind that it is only intended to direct attention to the predominant alkali or alkaline earth in the mineral, not to a.s.sert the absence of the others, which in most cases will be found to be present in minor quant.i.ty. Thus potash-feldspar (orthoclase) almost always contains a little soda, and often traces of lime or magnesia; and in like manner with the others. The terms "gla.s.sy" and "compact" feldspars only refer to structure, and not to species or composition; the student should be prepared to meet with any of the above feldspars in either of these conditions: the gla.s.sy state being apparently due to quick cooling, and the compact to conditions unfavourable to crystallisation; the so-called "compact feldspar" is also very commonly found to be an admixture of more than one feldspar species, and frequently also contains quartz and other extraneous mineral matter only to be detected by the microscope.

Feldspars when arranged according to their system of crystallisation are MONOCLINIC, having one axis obliquely inclined; or TRICLINIC, having the three axes all obliquely inclined to each other. If arranged with reference to their cleavage they are ORTHOCLASTIC, the fracture taking place always at a right angle; or PLAGIOCLASTIC, in which the cleavages are oblique to one another.

Orthoclase is orthoclastic and monoclinic; all the other feldspars are plagioclastic and triclinic.

MINERALS IN METEORITES.

That variety of the Feldspar Group which is called Anorthite has been shown by Rammelsberg to occur in a meteoric stone, and his a.n.a.lysis proves it to be almost identical in its chemical proportions to the same mineral in the lavas of modern volcanoes. So also Bronzite (Enstat.i.te) and Olivine have been met with in meteorites shown by a.n.a.lysis to come remarkably near to these minerals in ordinary rocks.

MICA GROUP.

With regard to the micas, the four princ.i.p.al species (Table 28.1) all contain potash in nearly the same proportion, but differ greatly in the proportion and nature of their other ingredients. Muscovite is often called common or potash mica; Lepidolite is characterised by containing lithia in addition; Biot.i.te contains a large amount of magnesia and oxide of iron; whilst Phlogopite contains still more of the former substance. In rocks containing quartz, muscovite or lepidolite are most common. The mica in recent volcanic rocks, gabbros, and diorites is usually Biot.i.te, while that so common in metamorphic limestones is usually, if not always, Phlogopite.

AMPHIBOLE AND PYROXENE GROUP.

The minerals included in Table 28.1 under the Amphibole and Pyroxene Group differ somewhat in their crystallisation form, though they all belong to the monoclinic system. Amphibole is a general name for all the different varieties of Hornblende, Actinolite, Tremolite, etc., while Pyroxene includes Augite, Diallage, Malacolite, Sahlite, etc. The two divisions are so much allied in chemical composition and crystallographic characters, and blend so completely one into the other in Uralite, that it is perhaps best to unite them in one group.

THEORY OF ISOMORPHISM.

The history of the changes of opinion on this point is curious and instructive.

Werner first distinguished augite from hornblende; and his proposal to separate them obtained afterwards the sanction of Hauy, Mohs, and other celebrated mineralogists. It was agreed that the form of the crystals of the two species was different, and also their structure, as shown by CLEAVAGE-- that is to say, by breaking or cleaving the mineral with a chisel, or a blow of the hammer, in the direction in which it yields most readily. It was also found by a.n.a.lysis that augite usually contained more lime, less alumina, and no fluoric acid; which last, though not always found in hornblende, often enters into its composition in minute quant.i.ty. In addition to these characters, it was remarked as a geological fact, that augite and hornblende are very rarely a.s.sociated together in the same rock. It was also remarked that in the crystalline slags of furnaces augitic forms were frequent, the hornblendic entirely absent; hence it was conjectured that hornblende might be the result of slow, and augite of rapid cooling. This view was confirmed by the fact that Mitscherlich and Berthier were able to make augite artificially, but could never succeed in forming hornblende.

Lastly, Gustavus Rose fused a ma.s.s of hornblende in a porcelain furnace, and found that it did not, on cooling, a.s.sume its previous shape, but invariably took that of augite. The same mineralogist observed certain crystals called Uralite (see Table 28.1) in rocks from Siberia, which possessed the cleavage and chemical composition of hornblende, while they had the external form of augite.

If, from these data, it is inferred that the same substance may a.s.sume the crystalline forms of hornblende or augite indifferently, according to the more or less rapid cooling of the melted ma.s.s, it is nevertheless certain that the variety commonly called augite, and recognised by a peculiar crystalline form, has usually more lime in it, and less alumina, than that called hornblende, although the quant.i.ties of these elements do not seem to be always the same.

Unquestionably the facts and experiments above mentioned show the very near affinity of hornblende and augite; but even the convertibility of one into the other, by melting and recrystallising, does not perhaps demonstrate their absolute ident.i.ty. For there is often some portion of the materials in a crystal which are not in perfect chemical combination with the rest. Carbonate of lime, for example, sometimes carries with it a considerable quant.i.ty of silex into its own form of crystal, the silex being mechanically mixed as sand, and yet not preventing the carbonate of lime from a.s.suming the form proper to it. This is an extreme case, but in many others some one or more of the ingredients in a crystal may be excluded from perfect chemical union; and after fusion, when the ma.s.s recrystallises, the same elements may combine perfectly or in new proportions, and thus a new mineral may be produced. Or some one of the gaseous elements of the atmosphere, the oxygen for example, may, when the melted matter reconsolidates, combine with some one of the component elements.