Allen and Crenshaw — Sulphides of Zinc. 341 



Art. XXXV. — The Sulphides of Zinc, Cadmium, and Mer- 

 cury; their Crystalline Forms and Genetic Conditions ; 

 by E. T. Allen and J. L. Crenshaw. Microscopic Study 

 by H. E. Merwin. 



Introduction. 

 Mineral genesis in its fullest sense includes not only a 

 knowledge of the limiting conditions outside of which a 

 mineral can not exist, but a knowledge of the particular pro- 

 cesses within the above limits by which the mineral is actually 

 formed in the earth's crust. Concerning the first half of 

 the problem, — the limiting conditions, the general relations 

 of composition, temperature and pressure are pretty well 

 understood, in so far as they have to do with equilibria. In 

 equilibrium studies, however, unstable forms are commonly 

 overlooked or avoided, but they must be taken account of in 

 mineral genesis because many minerals are unstable forms. 

 They differ from the unstable forms best known to chemists in 

 their greater inertness ; indeed, they often appear capable of 

 unlimited existence at ordinary temperatures, though there is 

 a potential difference between them and truly stable forms. 

 For unstable forms an upper limit of temperature may be set, 

 above which they can not exist. This is found experimentally 

 by a determination of the lowest temperature at which, under 

 the most unfavorable conditions, transformation into the stable 

 form can be followed. We say " under the most unfavorable 

 conditions," because the nature of the system in which the 

 ■ mineral undergoes transformation vitally affects the rate of the 

 latter. Heating alone, i. e., without the addition of any solvent 

 substance, is in general the condition most unfavorable to 

 change. The velocity of the change would then be at a mini- 

 mum and the temperature at which it first became noticeable 

 would mark the upper limit of the actual existence of the 

 mineral, above which it could never be formed.* This tem- 

 perature is evidently not a sharp point, but it is, at least in 

 some cases, a valuable limit in geology. 



The effect of pressure on reversible changes is comparatively 

 slight, in general, in systems containing no gaseous phase, 

 though it may be considerable on minerals formed at great 

 depths. In systems which contain a gaseous phase, the mass 

 law affords us a guide to the influence of pressure. It may 

 be noted here, by way of example, that in the formation of 

 sulphides with hydrogen sulphide, a variation in the pressure 

 of the latter only affects the quantity, not the nature of the 

 solid phase, and while it would also condition the concentration 



* Unstable forms may sometimes be obtained in the laboratory at temper- 

 atures where a slight disturbance is sufficient to effect their transformation, 

 but it is hardly to be supposed that anything of this sort occurs in nature. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIV, No. 202.— October, 1912. 

 23 



