Discoasterales
Calcareous nannofossils, whether monocrystalline with a stellate shape or multi-crystalline with stellate or rosette shapes, typically exhibit an optic axis that is vertical relative to the microscope stage in their plan view (natural settling view). When viewed in this orientation under a microscope, they appear to be in constant extinction (dim) because they do not double-refract light.
However, these nannofossils display birefringence when observed from a different angle or orientation (side view). This optical property causes asteroliths of the Discoasterales order to split incident light into two rays travelling at different velocities. As a result, they exhibit interference colours when viewed under XPL. In this alternative orientation, they show parallel extinction lines (extinction angle is 0°), meaning they go to extinction
position (dim) when their elongation axis aligns with the vibration directions of the polariser and analyser. Additionally, they demonstrate length-slow (+) elongation.
Understanding these optical characteristics is crucial for identifying and studying stellate or rosette-shaped calcareous nannofossils, providing valuable insights into their structure and composition.
Hay, W. W. 1977. Calcareous nannofossils. In: Ramsay, A. T. S. (Ed.) - Oceanic Micropalaentology. Academic Press, London 1055-1200.
