Nuclear Quadrupole Coupling Tensors of 27Al in Andalusite (Al2SiO5)

The nuclear quadrupole coupling tensors of ²⁷Al at the two crystallographically nonequivalent Al³⁺ sites in andalusite, Al₂SiO₅, have been determined from nuclear magnetic resonance measurements. For this, the Hamiltonian matrix for the mixed magnetic and electrostatic interaction was diagonalized and a minimization technique based on the least‐squares principle was used. The quadrupole coupling constants and asymmetry parameters are: $${\mathrm{Site\; Al}}_1{\mathrm{eQV}}_{\mathrm{zz}}\text{\hspace{0.17em}=\hspace{0.17em}15.6}\mathrm{MHz}\text{, η\hspace{0.17em}=\hspace{0.17em}0.08 (}\mathrm{octahedral}\mathrm{);}$$ $${\mathrm{Site\; Al}}_2{\mathrm{eQV}}_{\mathrm{zz}}\text{\hspace{0.17em}=\hspace{0.17em}5.9}\mathrm{MHz}\text{, η\hspace{0.17em}=\hspace{0.17em}0.70 (5‐}\mathrm{coordinated}\mathrm{).}$$ The site assignment which cannot be made from NMR data alone is based on point charge calculations (point multipole model). Previous assignments concluded from EPR data of Fe³⁺ and Cr³⁺ impurities at the Al³⁺ sites are confirmed. The principal axes of the magnetic Fe³⁺ and Cr³⁺ tensors deviate from the ²⁷Al tensor axes by 1°–10°.