The effects of stress on reactions accompanied by a gas or vapour phase are very great. The mere grinding of calcium car bonate in a mortar leads to the evolution of some carbon dioxide. Reactions which are inappreciable in the absence of stress may thus be greatly facilitated by application of stress. The reaction CaCO3+Si02 ± CaSiO3+CO2 is an important one in rock metamorphism. Under high uniform pressure the left side of the equation represents the stable assemblage, and calcite and quartz are found together. On the other hand, at high temperatures the reaction proceeds with the formation of wollastonite. The approximate equilibrium curve for this reaction has been calcu lated on the basis of the Nernst heat theorem. From the stand point of the phase rule, a mixture of calcium carbonate and silica represents a. three-component system and therefore under the conditions of metamorphism only three phases are possible. Below the transformation curve the pos sible three phase assemblages a r e a n d — Above the transformation curve the as semblages are — CO2 and Si02— (fig. 3). The occurrence together of calcite and quartz might therefore be used to indicate that the rock assemblage had not been heated above the transformation temperature, and in this way the mineral constitution of such a rock might be used as a geological thermometer. The effects of shearing stress are of a different order if the vapour phase is allowed to escape, for the reaction is driven in one definite direc tion leading to the production of wollastonite. In the choice of transition points adapted for the purposes of a geological ther mometer it is clear that much care is necessary and the condi tions of metamorphism realized.
The effects of temperature on reversible reactions are usually of a different order of magnitude from those of pressure. A large increase of pressure may be less effective in displacing equilibrium than a small increase of temperature. Nevertheless the effects of pressure are by no means negligible. In such reactions increase of pressure tends to displace the equilibrium in the direction in which the reaction is accompanied by decrease of volume. Thus, in a reacting system, high pressure favours the production of phases of greater density. This well known law of Le Chatelier (1885) was first applied to metamorphic processes by Lepsius (1893), but its importance was first clearly enunciated by Becke, whence it is frequently referred to as Becke's volume law. The knowledge of the specific volumes of minerals is largely gleaned from observations at ordinary temperatures, and under the con ditions of reaction the change in volume is not necessarily the same in magnitude, nor even in sign; moreover some of the re actions to which the law is applied are not definitely known to be reversible. Its application to mineralogical systems requires the exercise of considerable caution. Undoubtedly it is in the rocks of higher grades of metamorphism where enormous pressures are involved that the volume law is operative. Such minerals as
jadeite, pyrope, almandine, grossular, kyanite and staurolite are dense minerals represented only in rocks formed at high pressures, and arise under the operation of the volume law. The combination wollastonite-anorthite appearing in limestone xenoliths at volcanic centres, under the conditions of contact metamorphism or deep seated alteration, appears as a grossular-quartz assemblage: In dry melts, a system of the bulk composition of grossular con solidates as a mixture of the phases anorthite, wollastonite and gehlenite. Regarded as a reversible action: the formation of grossular is accompanied by a large decrease of volume. The sum of the molecular volumes of the phases of the right hand side of the equation is 31o, the molecular volume of grossular 26o. Clearly, pressure favours the formation of the garnet mineral.
Again glaucophane or jadeite appears in place of nepheline and albite under high pressure : the formation of garnet being accompanied by a diminution in volume of 17%.
In the more deep-seated regions metamorphism is effected under the influence of widespread magmatic activity. An intimate corn mingling of igneous and metamorphic rock is therefore charac teristic of these regions. Injection gneisses are produced by the lit par lit intrusion of igneous magma between the foliation planes of metamorphic rocks. The process is not always limited to a mechanical injection of material, but may be accompanied by an intense metasomatic action, in which solutions emanating from the magma react with the country rock to produce metasomatic schists and gneisses Recrystallization processes in metamorphic rocks take place in an essentially solid environment, and in distinction from igneous rocks no definite sequence or order of separation of crystals is to be traced. The characteristic structures and textures of meta morphic rocks have, therefore, a distinct significance. The term "structure" is used to express the genetic relationship of the com ponent minerals, while "texture" refers more explicitly to their stereometric arrangement. The typical structure produced by the growth of crystals in a solid environment is known as the crystal loblastic structure (Gr. (3Xacrravecv, to sprout). Since each grain grows in intimate contact with its neighbours, the form-develop ment of the individual crystals tends in general to be poor. Rounded crystals are thus very common. Different minerals, however, possess varying crystallization force, and some are able to assert their proper crystalline form against the resistance of their solid environment. The experiments of Becke and Day on the growth of crystals of alum under load, show that the force of crystallization may be very great, and indicate that the internal stresses set up during growth are of the same order of magnitude as the crushing strength of crystals themselves, and, indeed, as the forces brought into play during orogenic movements.