DIAMAGNETIC ANISOTROPY OF SHEETSILICATES

The diamagnetic properties of sheetsilicates are studied by measuring the magnetic orientation of micron-sized crystals. In the case of synt hetic phlogopite with the average diameter of 0.65 mum and the thickne ss of 0.2 mum, the alignment of the grains is achieved in the magnetic field below 35 kOe. The planes of the platy single-crystal grains wer e alignned parallel to the field when the alignment was achieved. The alignment of the grains is realized because the field-induced anisotro pic energy, caused by the diamagnetic anisotropy in the mineral's crys tal structure, exceeds thermal agitation energy in the applied field. By analyzing the field dependence of grain alignment, the diamagnetic anisotropy per formula unit, DELTAchi, of sheetsilicate mineral is est imated even if a large single crystal is not provided. The field-induc ed anisotropic energy is proportional to NDELTAchi where N is the numb er of molecules in the grain. A linear correlation has been found betw een the DELTAchi value and the F/OH mole ratio among the measured shee tsilicates. This correlation supports the assumption that the hexagona lly packed oxygen-layer in the crystal lattice induces the anisotropie s of the sheetsilicates.