PRESSURE IN FORMATION OF ROCKS AND MINERALS 737 



consequently the reasoning with regard to the shape of the melting- 

 curve is, mutatis mutandis, applicable here; with this difference, 

 however, that a maximum on the transformation curve is possible, 

 since there appears to be no necessity that the solid form stable at 

 the higher temperature should always have the greater specific 

 volume. On the other hand, nothing whatever can be predicted 

 as to the influence of pressure on monotropic (apparently irrever- 

 sible) transformations, such for example as marcasite->pyrite, or 

 aragonite->calcite; for such inversions take place whenever and 

 wherever their rate becomes appreciable. Consequently the tem- 

 perature region within which a monotropic inversion takes place 

 varies with the conditions, especially such as the presence of a 

 solvent; but there is no evidence that uniform pressure accelerates 

 such transformations at all."^ This conclusion is borne out by the 

 fact that high pressure acting on either marcasite or aragonite was 

 absolutely without effect in inducing the transformation to the 

 more stable form. In this connection, however, it is to be remarked 

 that it is possible that an inversion which is monotropic at one 

 pressure may at another pressure be enantio tropic; no actual case 

 of this is known, but this is not surprising in view of the small 

 amount of work hitherto carried out under a wide range of 

 pressures. 



Very frequently rise of temperature causes one crystal form to 

 change over into another (e.g., quartz -> tridymite -> cristobalite) ; 

 rise of pressure (at constant temperature) in many cases produces 

 a precisely analogous result — in other words, certain modifications 

 are really stable only at high pressure. It follows that investiga- 

 tion of pure substances over a wide pressure range will frequently 

 result in the discovery of hitherto unknown modifications of those 

 substances. A beautiful example of this is the substance water, of 

 which no fewer than five solid modifications are now known ;^ the 

 relations between these are given in Table II and also in Fig. i, 



' The influence of pressure on rate of reaction in condensed systems has been 

 investigated in very few cases: apparently it is very small, and may be either an accel- 

 eration or retardation. See E. Cohen and R. B. de Boer, Z. physik. Chem., LXXXIV 

 (1913), 41; E. Cohen and H. F. G. Kaiser, Ibid., LXXXIX (191 5), 338. 



2 P. W. Bridgman, Proc. Am. Acad., XL VII (1912), 439-558; for other examples 

 see Bridgman, Physic. Rev., Ill (1914), 153-203. 



