THE MINERALOGRAPHY OF THE FELDSPARS 285 



C. Temperature horizontal, inversion of orthoclase to microcline ipy^N. Solu- 

 bility line when orthoclase inverts to microcline. 

 N. Or 92 Ab 8, approximate. 

 M. Or 6 Ab 94, approximate. 



i. Or 72 Ab 28. 



q. Or 12 Ab 88. 



Legend for the diagram after Dittler: 

 Minimum point. Or 36 Ab 64. 



Makinen's legend: 



A. "Melting point" of orthoclase, 1190° C. 

 i?. Melting point of albite, 1180°. 

 C. Eutectic point 30 per cent of Or. 

 DE. Miscibility gap. Solidification of the melt. Greatly exaggerated. 

 ADGF. Stable field of homogeneous monoclinic solid solutions (orthoclase, 



sanidine, and soda-orthoclase) . 



BEIL. Stable field of homogeneous triclinic solid solutions (albite, anorthoclase) . 



DEIGH. Complex, two-phase field, of monoclinic potash feldspar and triclinic soda 



feldspar solid solutions (orthoclase-perthite, crypto- and microperthite) . 



GHF. Complex, two-phase field, of monoclinic and triclinic potash feldspar 



solid solutions (no examples found in natural feldspars) . ( ?) 

 FHK. Stable field of homogeneous triclinic potash feldspar solid solutions 

 (homogeneous microcline) . 

 HILK. Complex, two-phase field, of triclinic potash feldspar and triclinic soda 

 feldspar solid solutions (microcline-perthite and antiperthite) . 

 See text for criticism of Makinen's diagram. 



Marcos legend: 



a. Melting point of orthoclase. 



b. Melting point of albite. 



c. Eutectic point of monoclinic orthoclase-rich solid solution and triclinic albite- 

 rich solid solution. 



cf and dg. Boundaries of miscibilites. Solubility lines. 



h. Transition point monoclinic ^triclinic potash feldspars. 



i. Transition point monoclinictr:i:±;icilinic soda feldspars. 

 kdi. Boundaries of triclinic condition. 

 hci. Boundaries of monoclinic condition. 

 Interval hcidh. Transition interval. 



Whether the transition point i is situated above the melting point b has not been 

 determined. 



modification of Vogt's^ original (Fig. i, Part II). It shows a eutec- 

 tiferous system with the eutectic point at 42 per cent orthoclase, 

 58 per cent albite (plus anorthite), with inclined solubility curves. 



^J. H. L. Vogt, " Physikalisch-chemische Gesetze der KrystaUisationsfolge in 

 Eruptivgesteinen," Tsch. Min. Petro. Mitt., XXIV (1905), XXV (1906), and XXVII 

 (1908). 



