Albite - Orthoclase System

Ab - Or System 8/26/2011

• Important in the study of granitic rocks, with the addition of quartz, first approximation of a true granite • the appearance or topology of the system changes with pressure • the first petrologic system to be studied in detail (Tuttle and Bowen 1958)

Brock University © G.C. Finn

The Ab-Or System "Dry" Conditions At low pressures (“Dry” Conditions): 1) System displays complete solid solution 1200 Leucite Liquid between Or and Ab. 2) System displays a solvus, below which 1100 the feldspar solid solution ‘unmixes’. L + Leu + Or SS AbSS + L Or + L 1000 SS 3) System has a minimum, no eutectic. 4) The system is pseudo-binary, with 900 1 Feldspar Leucite appearing on the liquidus as a Solid Solution primary phase. 800 The pseudo-binary nature comes from the fact that the composition of Leucite 700 (KAlSi2O6) cannot be represented in terms of the two end member components Or Solvus (KAlSi3O8) and Ab (NaAlSi3O8) and the Temperature (°C) 600 presence of the three phase field L + Leu 2 Feldspars +OrSS. 500

Or 20 40 60 80 Ab

Brock University © G.C. Finn

The effects of increased pressure on the Ab-Or System "Dry" Conditions P = 1000 bars P = 2000 bars

1200 Leucite Liquid Liquid Liquid 1100 L + Leu + Or SS AbSS + L Leucite Leucite Or + L 1000 SS

OrSS + Ab + L SS Or 900 1 Feldspar L SS + AbSS + L Solid Solution L 800 1 Feldspar Solid Solution 1 Feldspar 700 Solid Solution Solvus Solvus Solvus

Temperature (°C) Temperature 600 2 Feldspars 2 Feldspars 2 Feldspars 500

Or 20 40 60 80 Ab Or20 40 60 80 Ab Or 20 40 60 80 Ab 1) Disappearance of Leucite 2) Lower melting temperature of end member components 3) Nature of the phase diagram changes, minimum disappears and is replaced by a eutectic.

Brock University © G.C. Finn

1 Ab - Or System 8/26/2011

Experimental results in the Or-Ab System (after Yoder et al. 1975)

P H2O = 5000 bars 1000 Liquid + H O vapour 876°C 2 900

Ab + L 800 OrSS + L SS + H2O vapour 695°C + H2O vapour 748°C

700 OrSS AbSS + H2O vapour + H2O vapour 600 2 feldspars + H O vapour

Temperature (°C) Temperature 2 500

400

Or10 20 30 40 50 60 70 80 90 Ab Weight Percent Ab Brock University © G.C. Finn

P = 2000 bars

P = 5000 bars H2O Liquid

Leucite 1000 1000

Liquid + H2O vapour

900 OrSS + AbSS + L Ab + L SS L 800 + H2O vapour OrSS + L + H O vapour 2 1 Feldspar OrSS 700 + H O vapour Solid Solution 2 AbSS + H O vapour 2 Solvus 600 2 feldspars + H2O vapour 2 Feldspars Temperature (°C) Temperature 500 500

400 Or 20 40 60 80 Ab

Lc + L 1000 P = 2 kbars Lc + Or + L SS Liquid

OrSS + L Phase Rule 900 Liquidus P + F = C + 1 m At m 800 P = 2 Solidus F = 1

700 1 FeldsparSS At c c P = 3 F = 0 Solvus 600 At x 2 Feldspars P = 2 F = 1 x 500

20 40 60 80 Or Ab

2 Ab - Or System 8/26/2011

Lc + L 1000 P = 2 kbars Lc + Or + L SS Liquid Or + L 900 SS

800

700 1 FeldsparSS

600 Pl 2 Feldspars Or

500