lOO REPORTS OF INVESTIGATIONS AND PROJECTS. 



valuable circumstantial evidence; but rather from known (several have now 

 been worked out) temperature relations existing between crystals of the same 

 composition but of different crystal form. 



One or two simple cases taken from the recent work of the Laboratory 

 will suffice to illustrate the point. The mineral CaO.SiOg melts readily at a 

 temperature a little over 1500° C. If it is cooled again and recrystallizes at 

 that temperature, a crystalline mineral known as pseudo-wollastonite results. 

 If it does not recrystalize promptly (and most minerals and mineral mixtures 

 do not), but cools quietly below 1200° and crystallizes there, wollastonite, a 

 familiar natural mineral, results. Chemically the two minerals are identical, 

 crystallographically they are different. The presence of original crystals of 

 wollastonite in a rock is therefore proof positive that its formation occurred 

 below 1200°. 



Some mineral compounds possess more than two crystal forms in the solid 

 state and thus receive further distinguishing marks long after the segregation 

 from the original magma is complete. Calcium orthosilicate, for example, 

 possesses three crystal forms,* magnesium metasilicate five,f pure silica 

 (quartz) six,t etc., and each is formed in a definite temperature region which 

 is measurable. Some, upon cooling, pass readily through several of these 

 states in succession ; others show much inertia in passing from one form to 

 another (inversion) and lag behind the temperature for a time; some of the 

 transitory states leave their permanent imprint upon the final form, while 

 others do not. Quartz, for example, which is the most conspicuous and 

 widely distributed of the minerals mentioned, offers in itself quite a scale of 

 temperatures of direct geologic application. Although the interrelation of 

 all the six forms (a and /? quartz, a and ^ tridymite, a and ^ cristoballite) has 

 not yet been completely worked out, at least four valuable geologic tempera- 

 tures have already been determined: (i) The transition (inversion) from a 

 to /? tridymite, occurring at 130° (reversible) ; (2) from a to j8 cristoballite 

 at 175° (reversible) ; (3) from a to /? quartz at 575° (reversible) ; (4) from 

 )8-quartz to ^-cristoballite at 800° (reversible). 



In the same way the relation of the bands in minerals which show zonal 

 structure, like the lime-soda feldspars, have a definite temperature signifi- 

 cance to which attention has already been directed § but upon which little 

 detailed work has yet been done. 



With the exception of quartz, these relations have merely been established 

 and the temperatures fixed as a part of the systematic study of the conditions 



* The lime-silica series of minerals. Am. Journ. Sci. (4), 22, 294, 1906. 



t Minerals of the composition MgSiOs; A case of tetramorphism. Am_. Journ. Sci. 

 (4), vol. 22, 437, 1906; Diopside and its relations to calcium and magnesium metasili- 

 cates. Am. Journ. Sci. (4), vol. 2y, 16, 1909. 



$ The binary systems of alumina with silica, lime, and magnesia. Am. Journ. Sci. (4), 

 vol. 28, 322, 1909. 



§ The isomorphism and thermal properties of the feldspars. PubHcation No. 31, Car- 

 negie Institution of Washington, p. 70. 



