Pseudo-pleochroic Calcite in Recrystallized Shell-Limestones

Limestones consisting of recrystallized shells of lamellibranchs, originally composed of aragonite, contain a brown-coloured calcite which is strongly pleochroic. Maximum “absorption’ corresponds to the extraordinary ray. The effect is shown to be a pseudo-pleochroism, caused by scattering of the extraordinary ray by inclusions, which follow the shell structure, in the calcite. Aragonite shells, which have not recrystallized, show a pseudo-pleochroism of similar colour but reversed sign. The inclusions originate in the conchiolin of the lamellibranch shell; their present nature is not known. Their progressive destruction by thermal metamorphism is described.