The nuclei of pleochroic haloes

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Abstract
Nuclei of pleochroic haloes in biotite from New Zealand granites are monazite, xenotime, occasionally apatite, but never zircon. Optical data for monazite, alpha 1.787, gamma 1.839, 12V 2-14 degrees , are comparable to that for monazite in nearby sands containing U 3 O 8 1.23 percent and ThO 2 2.32 percent. It is uncertain whether the apatite radioactivity is due to U or Th replacing Ca in the crystal lattice or to radioactive mineral inclusions. U and Th in the lattice of monazite and xenotime appears to result from similarity in size of the Y, Th, and U ions; the smaller size of the Zr ion allows limited substitution.

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