Abstract

Twenty-nine biotities corresponding to the formula K(MgxFeyAlz)Al1+z+Si3-zO10(OH)2, where z = 3-x-y, have been crystallized hydrothermally from oxide mixes. Syntheses were carried out at Pfluid = 1000 bars, PH2 = 100 bars, and T = 700°-770°C for the Fe3bearing biotites, and at PH2O = 1000-2000 bars, and T = 800°-850°C for the Fe-free biotites. Multiple regression analyses of the unit cell parameters for lM micas and of the optical indices of the refraction Yield:

 
a(Å)=5.406(1)0.089(2)xx+y0.056(3)z+0.022(4)zxx+yesd=0.002
 
nγ=1.686(1)0.103(2)xx+y0.029(3)z+0.037(4)zxx+yesd=0.001
 
where0xx+y1and0z0.75±0.1

Calculations of the α-tetrahedral rotation angle and the K-O bond length on the basis of an ideal trioctahedral mica show that for the most aluminous biotites the α rotations range from 12° to 14.7°, corresponding to K-O bond lengths of 2.89 to 2.77 Å respectively. If the smaller Na+ or Ca2+ ion is substituted for the K+ ion, the amount of A1 substitution and the amount of α rotation can be increased substantially. These data suggest that an important structural factor limiting the trioctahedral Al-substitution is the decreasing size of the interlayer site resulting from increased tetrahedral rotation.

Biotites synthesized at T> 700°C appear to be 1M or 3T polytypes. At lower temperatures, the Mg-biotites showed increasing amounts of what appears to be a 2M1 polytype in a 1M-2Ml mixture; thus, if the 2M1 polytype has a stability field, it probably lies at relatively low temperatures.

Although the substitutions Fe2+ ⇄ Mg2+ and Mg2+ + Si4+ ⇄ 2A13+ occur with essentially zero volume of mixing, the substitution Fe2+ + Si4+ ⇄ 2Al3+ has a net positive volume of mixing, This indicates that the solution properties of the Fe-Al biotites are not ideal and suggests that there could be octahedral ordering in the Fe-Al-rich biotites.

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