The Ternary System MgO-Al,0,-SiO v 317 



nary compound at this maximum is near 1 :1 :3 the maxi- 

 mum should be easily located ; on the other hand if near 

 2 :2 :5 it would be so near point 5 that it might not be 

 definitely established. As already stated, melting- tem- 

 peratures gave no decisive information concerning such 

 a maximum, therefore a supposedly favorable sample 

 (MgO 25, A1 2 3 20, Si0 2 55) was heated 18 hours at 1345° 

 to 1355° to determine if forsterite would appear in 

 recognizable form, as a means of settling the question of 

 a maximum. No forsterite was detected. Thus 2:2:5 

 must be approximately the composition of the ternary 

 compound separating near point 5. But before a more 

 definite conclusion is arrived at the properties of the 

 crystals of this compound separating in various parts 

 of the field must be considered, and such properties com- 

 pared with optical properties of crystalline aggregates 

 obtained by crystallizing at lower temperatures glasses 

 having the supposed composition of the compound. 



Faceted crystals of cordierite are best obtained from 

 melts in which it (or sillimanite) is the primary phase. 

 Such crystals can be studied to best advantage when they 

 occur in glass of considerable higher refractive index, 

 i. e., in melts containing less than 55 percent Si0 2 . All 

 the crystals observed, with the exception of those in one 

 quench, were stout six-sided prisms with basal termina- 

 tion and negative elongation. The prism angle, within 

 the limits of error of measurement, was 120°, and basal 

 sections gave a nearly or quite uniaxial negative inter- 

 ference figure. In the one exceptional case several 

 crystals were observed which gave unmistakable evidence 

 of orthorhombic symmetry, although their general habit 

 was just described. These crystals had the 120° prism 

 and the side and basal pinacoids corresponding to the 

 faces on all the other crystals ; they had also the front 

 pinacoid, and a second prism which bevels the edge 

 between the side pinacoid and the unit prism. 



The double refraction on the base is too small to give 

 the orientation of fi and 7 but a ||c. The refractive 

 indices of crystals from melts highest in Si0 2 (66%), 

 are a = 1-519, /3 and 7 =1-522. From melts containing 

 about 50% Si0 2 a =1-524, and 7 = 1-528. The dif- 

 ferences are not much greater than the error of deter- 

 mination, ±-002, but yet they may represent considerable 



