182 iV" L. Bowen — Crystallization — 



liquid was inclined to the horizontal because the crucible 

 became pinched very thin at one side and the liquid rose by 

 capillarity at that side. Below the line of amphibole marking 

 the surface the glass is entirely free from crystals for a con- 

 siderable distance. Pyroxene crystals then appear sparingly 

 and they increase in abundance downward. In the lower part 

 the crystals occur for the most part in clusters, though in- 

 dividual crystals are quite frequently discernible. The effect 

 shown in fig. 3 was obtained in 45 minutes. 



The Floating of Tridymite. 



Examples of crystals separating from silicate liquids heavier 

 than themselves are probably rare in nature. Orthoclase may, 

 perhaps, be lighter than some of the more basic magmas from 

 which it may form, such as essexitic magma. Sodalite is 

 possibly lighter than some of the magmas from which it crys- 

 tallizes. The importance of the rising of crystals in a magma 

 is, however, probably small. In order to illustrate its possibil- 

 ity in certain artificial melts, mixtures of the same general type 

 as those showing sinking of olivine and pyroxene crystals were 

 used. By adding a still greater proportion of silica, mixtures 

 from which tridymite is the first mineral to crystallize are 

 obtained. On account of its very low density tridymite is to 

 be expected to rise in such a liquid. A mixture of 85 per cent 

 of the pyroxene, diopside 75 per cent, MgSi0 3 25 percent, with 

 15 per cent Si0 2 was held at 1430°, at which temperature it 

 consists of 98 per cent liquid and 2 per cent tridymite. On 

 account of the increased viscosity of this more siliceous melt 

 the rising of tridymite must be expected to take place slowly. 

 After 3 hours no definite effect was discernible but after 54. 

 hours a little more than l cm from the bottom of the crucible 

 was entirely free from tridymite crystals, all having risen 

 into the upper 5-6 mm of the liquid. The crystals were of sub- 

 hexagonal, tabular form, about 0'2 mm in diameter and 0'06 mm 

 thick. 



In fig. 4 a section near, the top of the melt is shown. The 

 tridymite crystals are fairly abundant in this portion, though 

 they constitute only about 2 per cent of the charge, the whole 

 lower portion being entirely free from them. There is also a 

 crowding together of the crystals at the top of the section 

 which corresponds with the top of the liquid. The crystals in 

 the uppermost part are distinctly smaller than those lower 

 down owing to the fact that they rose through only a short 

 distance before coming to rest at the surface and had there- 

 fore less opportunity for growth. The relation is of the same 

 nature as that shown by the olivine crystals at the bottom in 

 fig. 1. 



