Abstract

Single crystals were selected from a suite of seven partly to highly disordered albites prepared by heating low albite (Ab99.7Or0.3An0.0) from Clear Creek, California, at temperatures of 750 to 1000°C for 6 to 19 d in a piston-cylinder device. Refractive indices (at 589 nm) and extinction angles were found to vary linearly relative to the degree of (Al,Si) order as determined from the reciprocal lattice angles a* and y* measured on X-ray precession photographs. Values for completely ordered low albite (LA) and values extrapolated from LA through our intermediate and high albites to the completely disordered endmember (called analbite, AA) are
End-member nα nβ nγ 2Vx X′∧[100]on(010) X′∧[100]on(001) Θ Ψ Φ 
AA 1.5276 1.5350 1.5363 45.0° 6.2° 9.4° 68.7° 128.5° 99.3° 
LA 1.5288 1.5329 1.5394 103.0° 20.8° 3.2° 84.2° 107.2° 88.8° 
End-member nα nβ nγ 2Vx X′∧[100]on(010) X′∧[100]on(001) Θ Ψ Φ 
AA 1.5276 1.5350 1.5363 45.0° 6.2° 9.4° 68.7° 128.5° 99.3° 
LA 1.5288 1.5329 1.5394 103.0° 20.8° 3.2° 84.2° 107.2° 88.8° 
The easily measured optic axial angle 2V varies nonlinearly with (Al,Si) order. It can be used to estimate structural state using the following equations:  
t1(t1o+t1m)=0.353(17)+1.014(37)sin2Vx,Δt1(t1ot1m)=0.353(39)+2.121(83)sin2Vx,
where t1o and t1m are the Al contents of the T1O and T1m tetrahedral sites, respectively, and Vx = (2Vx)/2.
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