292 Day and Shepherd — Lime-Silica Series of Minerals. 



orthosilicate in the presence of the eutectic for reasons already 

 explained (p. 267), but the eutectic or the compound is readily 

 measured by itself once the composition has been determined. 

 Applying the method to the 67|- per cent mixture, therefore, 

 it was found to melt very sharply at 2015°. Neither the 

 orthosilicate nor the 70 per cent CaO composition showed any 

 trace of fusion at this temperature. Since the microscopic 

 properties of the orthosilicate remain unchanged in the pres- 

 ence of an excess of lime or of silica, it follows that the 

 orthosilicate does not form solid solutions with either lime or 

 silica. Both eutectics will therefore continue up to the ortho- 

 silicate. Above the lime-orthosilicate eutectic the pyrometer 

 affords no further information regarding the melting point 

 curve. 



The line MN (fig. 3) represents the temperature at which 

 the inversion to /3-orthosilicate occurs. It will be noted that 

 the inversion is frequently delayed by superheating, especially 

 in the region remote from the eutectic, but it was always possi- 

 ble to show by quenching the material from above and below 

 these temperatures that the variation is merely due to the 

 inevitable lag of the reaction. Between 50 and 65 per cent 

 of lime, the two heat changes at MN and KL lie so close 

 together that it was very difficult to separate them. The 

 pyrometer shows two points plainly, but each is somewhat 

 influenced by the presence of the other. We were able, how- 

 ever, to distinguish them beyond possibility of confusion by 

 holding the furnace constant at 1425° and quenching the 

 charge in water. These conditions yield a-orthosilicate -f 

 pseudo-wollastonite, while if the temperature is held at 1390- 

 1400°, /3-orthosilicate+pseudo-wollastonite results. The inver- 

 sion temperatures are contained in Table IV. Mean values 

 lose much of their significance in determinations of inver- 

 sion temperatures where superheating can occur and are 

 accordingly omitted from the table. 











Table IV. 

















Inversion (3- 



Orthosilicate to 



a- Orthosilicate. 









Percentage 





















of CaO 



55-5 



57-0 



60-0 



62-5 650 



70 



73 



75 



80 



90 



Inversion 



407° 



1395° 



1404° 



1396° 1426° 



1409° 



1426° 



1415° 



1407° 



1433 



tempera- 



1414 



1412 



1411 



1397 1421 



1405 



1425 



1421 



1429 



1432 



ture. 



1403 



1419 



1411 



1405 1415 



1412 







1432 



1433 





1411 



1415 

 1406 

 1406 

 1404 



1398 

 1402 

 1404 

 1405 



1388 



1398 1414 

 1408 1412 

 1407 1413 

 1401 1417 

 1423 

 1411 









1425 





