Nuclear Magnetic Resonance of 27Al and 1H in Zoisite, Ca2Al3Si3O12(OH)

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Abstract
The electric field gradient tensors at the two nonequivalent aluminum sites in zoisite, Ca2Al3Si3O12(OH), have been determined by means of nuclear magnetic resonance. The quadrupole coupling constants and the asymmetry parameters are 8.05 MHz and 0.46 for site I and 18.50 MHz and 0.160 for site II. The second coupling constant is the largest value measured so far for 27Al. The 10.4‐MHz difference of the coupling constants is the greatest value known for Al in nonequivalent sites with the same number of nearest neighbors and reflects the excessive distortion of the aluminum–oxygen octahedron of site II. Furthermore, the directions of maximum and intermediate field gradients at site II are nearly parallel to the directions of the shortest and largest Al–O bond length, respectively. These facts seem to indicate that in a highly distorted Al–O octahedron the electric field gradient tensor is predominantly determined by the octahedron itself. The second moment of the proton resonance line has been measured. A comparison with calculated values confirms the tentative hydrogen position as determined by x‐ray diffraction.

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