to Calcium and Magnesium Metasilicates. 41 



any conditions and are excessively small. The photomicro- 

 graphs (Plate I), a-f\ illustrate the changes in the shape of the 

 etch figures thus produced on 110 in preparations ranging from 

 diopside to pure /3-MgSiO a . The etch figures on diopside (a 

 and o) are long spindle-shaped pits with an upper blunt termina- 

 tion. On an average their length is four times the width and 

 the angle a between the two sides at their point of junction at 

 the lower extremity is about 18°. These etch pits are similar 

 in every detail to those on 110 of natural diopside from Ala,* 

 Piedmont. Etch pits on the face 010 were also observed and 

 likewise resembled those on 010 of the natural mineral. — The 

 etch pits on 110 of the preparation 50 per cent MgSi0 3 (c) are 

 very similar to the figures on diopside, the angle a between the 

 two sides at the lower extremity being slightly greater perhaps. 

 On crystals of the composition 55 per cent MgSi0 3 (d) the etch 

 pits are noticeably wider and the angle a has increased to about 

 35°. This angle a gradually increases until at the limit of solid 

 solution it measures approximately 50° {f). The relation of the 

 length of the etch pits to their width changes from about 4 : 1 in 

 diopside to 3 : 1 in the 55 per cent and about 5 : 3 at the limit as 

 represented by the etch figures of the 75 per cent preparation, 

 which were chosen in place of those of the 67 per cent because 

 of the sharper definition of the particular figures photographed. 

 The transitional changes in the shape of the upper portion of 

 the etch figures are also characteristic. The etch figures on 

 the cleavage faces of /3-MgSi0 3 (<?) are exceedingly small and 

 triangular in shape ; the angle a measures 55°-60° and the rela- 

 tion of length to width is about 3 : 2. Other details of the etch 

 figures appear in Plate I and the evidence from all view- 

 points tends to strengthen the assertion, that in this case of 

 limited or incomplete isomorphism the character of the etch 

 figures does change continuously with increasing MgSi0 3 , and 

 in the direction of the type of the etch figures of pure /3-MgSi0 3 . 



It is of interest to note that the /3-MgSi0 3 , the low tempera- 

 ture form, takes up very little if any diopside in solid solution. 

 This may be due to the fact that the high temperature-, a-form, 

 is orthorhombic and therefore would have less tendency to take 

 up the diopside molecule. 



On the calcium side of diopside the crystal miscibility in the 

 series is very slight, diopside taking up only small amounts 

 of the calcium metasilicate. Wollastonite, on the other hand, 

 can absorb up to 17 per cent of diopside and still remain homo- 

 geneous, whereas pseudo-wollastonite takes only 1 per cent at 

 most of diopside in solid solution. ]N~o satisfactory explanation 

 has been found to account for these differences in the behavior 

 of the different compounds of this series. 



* Compare E. A. Daly, Proc. Amer. Acad, of Arts and Sciences, xxxiv, 

 5373-428; pi. iv, No. 18, 1899. 



