432 K T. Allen, F. K Wright and J. K. Clement— 



monoclinic form is obtained; hence we conclude that the Bishop- 

 ville meteorite was probably cooled rather rapidly from a high 

 initial temperature. The occurrence of similar intergrowths of 

 the same minerals in many other meteorites indicates that the 

 above mode of formation is a general one, though ferrous sili- 

 cate, which is generally present, would lower the temperature 

 of crystallization. 



Occurrence of unstable forms of the metasilicate in nature. — 

 Since the monoclinic magnesian pyroxene is the stablest form, 

 the question naturally arises, why does it not occur more fre- 

 quently in nature. The probable explanation is that the mag- 

 nesium silicate of nature generally crystallized from solutions 

 or magmas, the temperature and viscosity of which conditioned 

 the formation of the enstatite or amphibole. 



In the foregoing, it has been proved that under atmospheric 

 pressure the monocline pyroxene is the most stable form of 

 magnesium silicate, and it has been shown how various solvents 

 may transform the other polymorphs into this one. Yet it is a 

 very common thing for unstable forms of enantiotropic, as 

 well as monotropic, substances to crystallize first* from solu- 

 tion, whether we start out with a stable or unstable form, pro- 

 vided the solution is not in contact with the stable solid; and 

 in one instance, that of mercuric iodide,f crystals of the unsta- 

 ble yellow variety sometimes form even in the presence of 

 nuclei of the red (stable) form, though they soon pass over into 

 the latter. 



"We had already found that the unstable amphibole forms 

 from water solutions at temperatures of 375°- 475°, and it 

 seemed worth while to make the attempt to produce enstatite as 

 well as the orthorhombic amphibole from silicate (magmatic) 

 solutions at higher temperature. We used for these experiments 

 portions of 50 to 60 grains, the solutions consisting of magne- 

 sium silicate mixed with about 10 per cent of its weight of vari- 

 ous other substances. The results are recounted briefly below: 



1. 40 grams magnesium silicate and 5 grams of ferric oxide were 

 fused in a Fletcher furnace. The heat was then turned off 

 and the crucible allowed to cool in the covered furnace in 

 which, as the walls are thick, the temperature falls at such a 

 moderate rate that the pure magnesium silicate crystallizes 

 almost entirely in the monoclinic form. A microscopic exam- 

 ination showed that the ferric oxide had not greatly influ 

 enced the crystallization. 



* Eegarding the crystallization of suphur, see Gmelin-Kraut, Handbuch 

 der Chemie, vol. 1, part 2, p. 155; and for similar facts about phosphorus, 

 see vol. 1, part 2, p. 10. See also O. Lehmann, Molekular-Physik, vol. 1, 

 p. 193, for many other instances of this kind. 



f Kastle and Keed, Am. Chem. Journ., xxvii, 217, 1902. 



