A quantitative study of pleochroic haloes. V. The genesis of haloes

There appear to be at least six distinct, elementary types of haloes, viz. the familiar uranium and thorium haloes and the four types designated A, B, C and D, which are fully described in the preceding paper of this series. It is the purpose of the present paper to discuss the genesis of these various haloes. The six types may be divided naturally into two classes according to the nature of the parent elements. By the term “parent” is meant that radioactive element which was first segregated in the halo nucleus during the process of mineral formation. The later disintegration of this and possibly of succeeding elements with emission ofα -particles gave rise to a halo with one or more rings. The parents of the uranium and thorium type haloes, U and Th, have periods which are very long, even when measured on a geological time scale. If once uranium or thorium be deposited in the halo nucleus, secular radioactive equilibrium with later members of the radioactive family concerned would be established relatively quickly. Hence all theα -particles from the family will have been emitted from the halo nucleus almost uniformly throughout the whole subsequent existence of the nucleus and of the surrounding biotite. The result is the familiar six-ringed halo of uranium or the five-ringed halo of thorium. Moreover,α -particles are being emitted from such halo nuclei at the present time, and the haloes are still in a process of development, becoming slowly darker as time goes on.