THE 



AMERICANJOURNALOFSCIENCE 



[FOURTH SERIES.] 



Art. I. — Diopside arid its Relations to Calcium and Magne- 

 sium Metasilicates ; by E. T. Allen and W. P. White. 

 With Optical Study : by Feed. Eugene Wright and 

 Esper S. Larsen. (With Plate I.) 



The pyroxenes were chosen some time since as a good 

 subject for laboratory investigation, both by reason of their 

 geologic importance and becanse their comparative stability 

 and simplicity of composition seemed to offer relatively little 

 difficulty in their synthesis and study. Diopside is the sim- 

 plest of this group of minerals, but before even this could be 

 studied satisfactorily, a detailed investigation of both calcium 

 and magnesium metasilicates was found necessary, a full 

 account of which has been given elsewhere.* Here it will be 

 sufficient to state that calcium silicate exists in two crystal 

 forms, one of which, the mineral wollastonite (/3-form), is stable 

 up to about 1190°, when it reverts to a pseudo-hexagonal form 

 (a-form) which melts at 1512°. The case of magnesium metasil- 

 icate is more complex. There is one form (monoclinic) strongly 

 resembling the pyroxenes, both optically and crystallographic- 

 ally, which is stable up to 1365° (/3-form). Here it passes 

 over into an orthorhombic form (a-form) recalling forsterite 

 (Mg 2 Si0 4 ) in its habit and optical properties. It melts at 

 1524°. Three other forms exist, viz., the minerals enstatite, 

 kupff'erite, and a monoclinic amphibole similar to the latter. 

 All are monotropic, and change into the /3-form when heated 

 to a sufficiently high temperature. 



The question of the relation of diopside to its component 

 salts, calcium and magnesium silicate, we undertook to settle 

 by determinations of the melting point and specific volume 



* Allen, White and Wright, this Journal, xxi, 89, 1906; Allen, Wright 

 and Clement, this Journal, xxii, 385, 1906. Their form, then undiscovered, 

 is described for the first time in these pages. 



Am. Jour. Sci.— Fourth Series, Vol. XXVII, No. 157.— January, 1909. 

 1 



