338 P. Eskola — Silicates of Strontium and Barium. 



the measurements failed to reveal any deviation from 

 hexagonal symmetry, I shall provisionally describe them 

 as hexagonal, although the isomorphism with the calcium 

 metasilicate makes it probable that the strontium metasi- 

 licate is pseudohexagonal and really monoclinic, in that 

 case belonging to the domatic, or monoclinic hemihedral 

 class. 



Fig. 2. 



Fig. 3. 



Fig. 4. 



Figs. 2, 3, 4.— Crystals of SrO.Si0 2 . 



In examining these crystals with a binocular micro- 

 scope one was found that was larger (about 0.5 mm in 

 diameter) and better than the rest, but it was incomplete, 

 having only faces in two pyramidal zones developed. 

 These zones were identical, each combined of three 

 different pyramids, p( 1011), o(4045) and w(2021) (fig. 4). 

 While searching for evidence of monoclinic symmetry I 

 measured a number of crystals, but unfortunately they 

 were too imperfect to prove anything positively. Often 

 the pyramidal faces were quite curved in their zones so 

 that the signals appeared as lines instead of points. All 

 the forms observed in the best crystals were, however, 

 also identified in other crystals, and some of them at the 

 same_ time in three pyramidal zones. A further form 

 t (5051) was found to be very common. 



The following data are based on the best crystal, except 

 for the last named form t. 



The crystals separating from silicate melts and 

 embedded in glass usually were bipyramidal in habit, 

 with large basal planes, and thinner than those obtained 

 from the chloride melt. 



In summarizing the evidence regarding the crystal 

 form of SrO.Si0 2 , we may state that, from its solid 

 solubility and apparent isomorphism with a-CaO.Si0 2 , it 

 might be expected to be monoclinic, but actually the 

 crystals agree so closely with the dihexagonal pyramidal 



