astonishing completeness in the history of individual products. In each class of rock crystallization follows a definite course.Sequence of Crystallization. The first minerals to separate belong to a group known as the minor accessories; this includes zircon, apatite, sphene, iron oxides; then follow in order olivine, augite, hornblende, biotite, plagioclase, felspar (beginning with the varieties most rich in lime and ending with those which contain most soda), orthoclase, microcline and quartz (with micropegmatite). Many exceptions to this rule are known; the same mineral may crystallize at two different periods; two or more minerals may crystallize simultaneously or the stages in which they form may overlap. But the succession above given holds in the vast majority of cases. Expressed in this way: the more basic minerals precede the less basic; it is known as Rosenbusch’s law.
Types of Structure.—In some rocks there seems to be little tendency for the minerals to envelop one another. This is true of many gabbros, aplites and granites (Pl. III, fig. 7). The grains then lie side by side, with the faces of the latter moulded on or adapted to the more perfect crystalline outlines of the earlier. More commonly some closer relationship exists between them. When the smaller idiomorphic crystals of the first-formed are scattered irregularly through the larger and less perfect crystals of later origin, the structure is said to be poikilitic (Gr. ποικίλος, many-coloured, mottled).Poikilitic. A variety of this, known as ophitic (Pl. III, fig. 6), is very characteristic of many dolerites and diabases, in which large plates of augite enclose many small laths of plagioclase felspar. Biotite and hornblende frequently enclose felspar ophitically; less commonly iron oxides and sphene do so. In peridotites the “lustre-mottled” structure arises from pyroxene or hornblende enveloping olivine in the same manner (Pl. III, fig. 8). In these cases no crystallographic relation exists between the two minerals (enclosing and enclosed).
But often the surrounding mineral has been laid down on the surface of the other in such a way that they have certain crystalline faces or axes parallel to one another.Parallel Growths. This is known as parallel growth. It is best seen in zoned crystals of plagioclase felspar, which may range in composition from anorthite to oligoclase, the more acid layers being deposited regularly on the surfaces of the more basic. Biotite and muscovite, hornblende and augite, enstatite and diallage, epidote and orthite, very frequently are associated in this way.
When two minerals crystallize simultaneously they may be intergrown in “graphic” fashion. The best example is quartz and orthoclase occurring together as micropegmatite (Pl. II, figs. 6 and 8).Graphic intergrowths. The quartz forms angular growths patches in the felspar, which though separated have the same crystalline orientation and one position of extinction, while the felspar on its part behaves in the same way Two porous crystals thus interpenetrate but the scattered parts of each mineral maintain their connexion with the others. There may be also a definite relation between the crystalline axes of the two crystals, though this is not known in all cases. Augite also occurs in graphic inter growth with hornblende, olivine and felspar; and hornblende, cordierite, epidote and biotite in graphic intergrowth with quartz.
Physical Chemistry of Igneous Rocks.—The great advances that have been made in recent years in our knowledge of physical chemistry have very important bearings on petrological investigations. Especially in the study of the genesis of igneous rocks we anticipate that by this means much light will be thrown on problems which are now very obscure and a complete revolution in our ideas of the conditions which affect crystallization may yet be the con sequence Already many important results have been gleaned. As yet little work of an exact and quantitative nature has been done on actual rocks or on mixtures resembling them in composition, but at the Carnegie Institution in Washington, an elaborate series of experiments in the synthesis of minerals and the properties of mixtures of these is being carried on, with all the refinements which modern science can suggest. The work of Doelter and of Vogt may also be mentioned in this connexion. At the same time the mathematical theory of the physical processes involved has received much attention, and serves both to direct and to elucidate the experimental work.
A fused mixture of two minerals may be regarded as a solution of one on the other. If such a solution be cooled down, crystallization will generally set in and if the two components be independent (or do not form mixed crystals) one of them may be expected to start crystallizing.Magmas as Solutions. On further cooling more of this mineral will separate out till at last a residue is left which contains the two components in definite proportions. This mixture, which is known as the eutectic mixture, has the lowest melting-point of any which can be formed from these minerals If heat be still abstracted the eutectic will consolidate as a whole; its two mineral components will crystallize simultaneously At any given pressure the composition of the eutectic mixture in such a case is always the same.
Similarly, if there be three independent components (none of which forms mixed crystals with the others), according to their relative amounts and to the composition of the eutectic mixture one will begin to crystallize; then another will make its appearance in solid form, and when the excess of these has been removed, the ternary eutectic (that mixture of the three which has the lowest melting-point) will be produced and crystallization of all three components will go on simultaneously
These processes have without doubt a very close analogy to the formation of igneous rocks Thus in certain felsites or porphyries which may be considered as being essentially mixtures of quartz and felspar, a certain amount of quartz has crystallized out at an early period in the form of well-shaped porphyritic crystals, and thereafter the remainder of the rock has solidified as a very fine-grained, cryptocrystalline or sometimes micrographic ground mass which consists of quartz and felspar in intimate intermixture. The latter closely resembles a eutectic, and chemical studies have proved that within somewhat narrow limits the composition of these felsitic ground-masses is constant.
But the comparison must not be pushed too far, as there are always other components than quartz and felspar (apatite, zircon, biotite and iron oxides being the most common), and in rocks of this type the gases dissolved in the magma play a very important part. As crystallization goes on, these gases are set free and their pressure must increase to some extent. Moreover, the felspar is not one mineral but two or perhaps three, there being always soda felspar and potash felspar and usually also a small amount of lime felspar in these porphyries.
In a typical basic rock the conditions are even more complex. A dolerite, for example, usually contains, as its last products of crystallization, pyroxene and felspar. Of these the latter consists of three distinct species, the former of an unknown number; and in each case the can form mixed crystals, to a greater or less extent with one another. From these considerations it will be clear that the properties of solutions of two or three independent components, do not necessarily explain the process of crystallization in any igneous rock.
Very frequently in porphyries not only quartz but felspar also is present in large well-formed early crystals. Similarly in basalts, augite and felspar may appear both as phenocrysts and as components of the ground-mass. As an explanation of this it has been suggested that supersaturation has taken place. We may suppose that the augite which was in excess of the proportion necessary to form the felspar-augite, eutectic mixture, first separated out. When the remaining solution reached the eutectic composition the felspar did not at once start crystallizing, perhaps because nuclei are necessary to initiate crystal-growth and these were not at hand; augite went on crystallizing while felspar lagged behind. Then felspar began and as the mixture was now supersaturated with that mineral a considerable amount of it was rapidly thrown out of the solution. At the same time there would be a tendency for part of the augite, already crystallized, to be dissolved and its crystals would be corroded, losing their sharp and perfect edges, as is often observed in rocks of this group. When the necessary adjustments had been made the eutectic mixture would be established and thereafter the two minerals would consolidate simultaneously (or nearly so) till crystallization was complete.
There is a good deal of evidence to show that supersaturation is not unimportant in igneous magmas. The frequency with which they form glasses proves that under certain conditions the molten rocks are highly viscous. Much will depend also on the presence, accidental or otherwise, of nuclei on which a mineral substance can be deposited. It is known that minerals differ in their tendency to crystallize, some doing so very readily while others are slow and backward. The rate at which crystallization goes on depends on many factors, and there are remarkable differences in this respect between minerals.
On the other hand, there is plenty of evidence to show that supersaturation, though probably one of the causes, is not the principal cause of the appearance of more than one mineral in two generations of crystals In some of the quartz-porphyries, for example, there are phenocrysts not only of quartz and felspar but also of micropegmatite. These prove that quartz and felspar were not crystallizing successively or alternately but simultaneously.
The great majority of the minerals found in igneous rocks are not of simple composition, but are mixtures of various elementary minerals in very different proportions This enormously complicates the theoretical problems of consolidation. It has been found, for example, that in the case of three minerals—one of which is independent, while the two others can form mixed crystals—there is a large number of possible sequences; and, what is very important, one mineral may separate out entirely at an early stage, or its crystallization may be interrupted and not continuous. The ternary eutectic, which is produced by a mixture of three independent minerals, may not in such a case be the last substance to crystallize, and may not be present at all. This is very much in accordance with the observed facts of petrology; for usually in a rock there is one mineral which indubitably was the last of all to finish crystallizing and contained no appreciable quantity of the others.
As yet we know little about such important questions as the composition of the eutectic mixtures of rock-minerals, their latent heat of fusion, specific heats, mutual solubilities, inversion temperatures, &c. Until we are in possession of a large body of accurate information on such points as these the theoretical treatment of
