An Introduction to the Study of Meteorites / With a List of the Meteorites Represented in the Collection

by British Museum . Department of Mineralogy; L. Fletcher

Cooling of fused mixtures and of solutions.

35. Though meteoric iron has been at some time, presumably, in a state of fusion, and its present structure is a result of the particular circumstances of the cooling of the liquid and afterwards solid material, attempts to produce such structures by the cooling of fused meteoric iron or artificial mixtures of nickel and iron have not yet been successful. It will be useful, therefore, to consider briefly some of the manifold changes which are found to take place during the passage of fused mixtures and of solutions to the solid state, and during the cooling of such solids to ordinary temperatures.

If a fused mixture of antimony and bismuth is allowed to cool, the solid which first separates is neither pure antimony nor pure bismuth, but a material which has a percentage composition depending on, though not identical with, that of the original mixture. The temperature for the beginning of the solidification is different for different proportions of the two metals, and is intermediate between 622° and 268°, the solidifying temperatures of antimony and bismuth, respectively; it approaches the latter more and more closely as the percentage of the bismuth is increased. The solid first separated is somewhat richer in antimony than the original mixture; the still fused part, therefore, is somewhat richer in bismuth than before, and does not begin to solidify till a lower temperature is reached; the temperature thus gradually falls, instead of remaining constant, during the solidification. In the cooling of such fused mixtures the changing composition of the part still fused has for effect a changing composition of the solid already separated; whence the slower the cooling of the fused material, the greater is the homogeneity of the final solid.

Eutectic mixtures.

A fused mixture of silver and copper behaves in a different way. When the percentage weight of the silver is 72, and that of the copper, therefore, is 28, solidification begins, not at a temperature between 960° and 1083°, the solidifying temperatures of silver and copper, respectively, but at a temperature below both, namely, 770°. The solid which first separates has the same percentage composition as the original mixture; the part still fused has thus itself the same percentage composition as before, and continues to Cooling of fused mixtures and of solutions. solidify at the same temperature, and in the same way, until the solidification is complete. Such a mixture, having a definite composition and a definite temperature of solidification, was for a time regarded as a definite chemical compound with a complex chemical formula, but on microscopic examination the resultant solid is found to be heterogeneous; minute particles of the silver and copper are seen to lie side by side, the particles being granular or lamellar in form according to the circumstances of the cooling. If the percentage of silver is different from 72, whether it be higher or lower, the solidification begins at a higher temperature than 770°; whence the mixture containing 72 per cent. of silver has been conveniently termed eutectic (i.e. very fusible); the term was suggested by Prof. F. Guthrie,[16] to whom our knowledge of the existence of such mixtures is due.

36. When the silver is in excess of 72 per cent., the excess of silver gradually collects together and solidifies at various parts of the cooling fused mass; the still fused portion thus gradually becomes poorer in that metal, and the temperature, instead of remaining constant, gradually falls during the separation of the solid. At length the percentage of silver in the fused portion falls to 72 per cent. and the temperature to 770°; the solid which now begins to form is no longer pure silver, but a material containing 72 per cent. of that metal; and it continues to have the same percentage composition as the surrounding liquid, and the temperature of solid and liquid to be 770°, until the solidification is complete. The final solid thus consists of blebs of silver scattered through a fine groundmass of eutectic mixture of silver and copper. Similarly, if the copper is in excess of 28 per cent., the final solid consists of blebs of copper scattered through a fine groundmass of eutectic mixture of silver and copper.

If the two metals are copper and antimony, instead of copper and silver, the results are more complicated; for the first two metals are capable of combining together to form a definite chemical compound represented by the formula Cu₂Sb, and each of the metals forms a eutectic mixture with the latter. According to the percentage composition of the original mixture, the solid which first separates during cooling from fusion may be either copper or antimony or the compound Cu₂Sb; the separation continuing, and the temperature falling, until the first eutectic proportion and its corresponding temperature are reached.

Cooling of solutions.

37. Analogous results are obtained during the cooling of solutions; for instance, during the cooling of a solution of sodium chloride (common salt) in water. A solution containing 23·5 per cent. of sodium chloride begins to solidify at -22° C.; the separating solid is not simple sodium chloride or simple ice, but has the same percentage composition as the original solution, and thus the temperature remains -22° until the whole material has become solid. On microscopic examination the solid is seen to be heterogeneous, and to consist of small particles of sodium chloride and ice lying side by side. If the percentage of sodium chloride is different from 23·5, whether higher or lower, solidification begins before the temperature has fallen to -22°. The characters of this particular solution are thus closely analogous to those of the eutectic mixtures described above. If the sodium chloride exceeds 23·5 per cent., the excess of sodium chloride begins to separate, and solidify, at various parts of the liquid, at a temperature higher than -22°; it continues to separate, and the temperature to fall, until the proportion of sodium chloride in the residual liquid is reduced to 23·5 per cent. and the temperature to -22°. Afterwards the separating solid has the same composition as the residual liquid (23·5 per cent. of sodium chloride), and the temperature remains constant, until the residual liquid has been wholly transformed into a solid fine-grained mixture of sodium chloride and ice. The final solid thus consists of large particles of sodium chloride dispersed through a fine groundmass consisting of eutectic mixture of sodium chloride and ice. Similarly, if the water is in excess of 76·5 per cent., the final solid consists of large particles of ice dispersed through a fine groundmass consisting of eutectic mixture of sodium chloride and ice.

The results of the cooling of a solution of ferric chloride are still more complicated; for this substance enters into chemical combination with water, and in no fewer than four different proportions. The solid which first separates from the cooling solution may thus, according to the percentage of ferric chloride, be either ferric chloride or water, or any one of the various compounds of the two; and to each pair of compounds nearest to each other in composition corresponds a different eutectic mixture and a different temperature for its formation.