Physical properties of some synthetic Fe-Mg-Al trioctahedral biotites

Twenty-nine biotities corresponding to the formula K(Mg_xFe_yAl_z)Al_1+z+Si_3-zO₁₀(OH)₂, where z = 3-x-y, have been crystallized hydrothermally from oxide mixes. Syntheses were carried out at P_fluid = 1000 bars, P_H2 = 100 bars, and T = 700°-770°C for the Fe₃bearing biotites, and at P_H2O = 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:

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-2M_l 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.