The crystal structures of titanian chondrodite [Mg3.99Fe0.57Mn0.01Ni0.01Ti0.42(SiO4)2OH1.15O0.85; a = 4.7271(7), b = 10.318(1), c = 7.9053(9)A, α = 109.333(8)°, P21/b, Z = 2] and titanian clinohumite [Mg7.44Fe1.09Mn0.02Ni0.02Ti0.43(SiO4)4OH1.14O0.86; a = 4.745(1), b = 10.283(2), c = 13.699(3)A, α = 101.00(2)°, P21/b, Z = 2] from the Buell Park (Arizona) kimberlite have been refined by full-matrix least-squares methods to R = 0.035 and 0.026, respectively. The results have established that Ti in the both structures is exclusively concentrated in the M(3) site located in Mg(OH)O layers. This situation explains the fact that attainable substitution of Ti for Mg in chondrodite is higher than that in clinohumite.

The final difference Fourier synthesis of titanian clinohumite has brought out hydrogen positions, H and H', which are related to each other by inversion, the separation between H and H' being 0.87A, and O-H = 1.02A. This feature implies that if the positions were fully occupied by hydrogen atoms as in the OH end-member, there would arise a strong hydrogen-hydrogen repulsion. The substitution, Mg + 2(OH) → Ti + 2(O), that reduces the number of OH groups in the structure, would therefore play a role in reducing the effect of repulsion to stabilize the structure, thus explaining the non-occurrence in nature of OH end-members of the humite-group minerals; the substitution of F for OH would then play the same role.

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