Andersen — System Anorthite-Forsterite-Silica. 411 



with the liquid at the higher temperature also remain un- 

 changed, unless they pass through inversion points. 



In Tables 2, 3, 4, and 5 those of the quenching experiments 

 which have any direct bearing on the determination of in- 

 variant points, boundary curves and liquidus surfaces are listed. 

 The data on the forsterite-silica system are given in detail in a 

 previous paper." 



The following abbreviations are used in the tables and some- 

 times in the text : 



An = Anorthite (CaAl o Si 2 8 ). 



Fo = Forsterite (Mg 2 SiG 4 ). 



En = Enstatite (MgSi0 3 without reference to modification). 



CI en = Clinoenstatite (MgSi0 3 ). 



Tr = Tridymite (Si0 2 ). 



Cr = Cristobalite (SiO„). 



Sp = Spinel (MgAl 2 4 ). 



Gl = Glass. 



II. Description of Solid Phases. 



A very important part of work like the present lies in the 

 identification of the solid phases occurring in the different mix- 

 tures at different temperatures. This identification has to be 

 based almost entirely upon microscopic examination of the 

 quenched products. It is therefore essential first to know in 

 detail the optical properties of the phases that are possible in 

 the mixtures considered and then to be able to redetermine 

 such properties with a sufficient degree of accuracy on the fine- 

 grained material obtained from the quenchings. 



In the present case all the solid phases occurring have been 

 previously described in papers from this laboratory. Their 

 properties have been determined on chemically pure material. 

 As to the recognition and measurement of these optical 

 properties on fine-grained powder under the microscope the 

 recent development of microscopy^ has made this compara- 

 tively easy, although it is still a matter requiring time and 

 patience. 



In the following descriptions of the stable, solid phases of 

 the anorthite-forsterite-silica system, the thermal, crystallo- 

 graphic and optical properties are first briefly reviewed with 

 reference to the earlier determinations made in this laboratory 

 and then general descriptions of the phases as they appear in 

 the present system are given. 



*N. L. Bowen and Olaf Andersen, this Journal (4), xxxvii, 487, 1914. 



f For descriptions of the most advanced methods see : F. E. Wright, The 

 Methods of Petrographic-Microscopic Research, Publ. No. 158, Carnegie 

 Institution of Washington, 1911. 



