bright/dark between cross-polars. N.B. A coccolith or part of a
coccolith can only appear non-birefringent in one orientation (when the
c-axis is vertical), so strictly these terms should not be used without explicit
description of specimen orientation; e.g. "discoasters are
non-birefringent in plan view".
|Extinction-figure ||appearance of a specimen in cross-polarized light, particularly pattern of isogyres.
|Isogyre ||dark line in cross-polarized light caused by elements in extinction.
|North/South, East/West ||orientations relative to the microscope body.
|Gypsum plate||accessory plate that gives a first-order red (approx. 1 λ) interference color with crossed polars when inserted at 45° to the polarizer and analyzer. Also called sensitive tint plate, or 1 lambda (1λ) plate. The standard orientation for use of the plate is with the slow direction (indicated by a double-headed arrow on the plate) oriented NE-SW. NB Calcite is a length-fast mineral hence constructive interference occurs when the c-axis is perpendicular to the slow direction of the accesory plate. [added 2017]|
|Length fast||c-axis oriented parallel to length, and so giving blue colour with gypsum plate in standard orientation when aligned NW-SE. May be applied to a single element, a structure (e.g. spine) or an entire nannofossil. [added 2017]
||Length slow||c-axis oriented perpendicular to length, and so giving yellow colour with gypsum plate in standard orientation when aligned NW-SE. May be apllied to a single element, a structure (e.g. spine) or an entire nannofossil. [added 2017]
|Compound ||formed of several crystal-units. E.g. Micula, Discoaster.
|Pseudo-monocrystalline ||formed of several crystal-units with parallel c-axes, but non-parallel a-axes. E.g. Discoaster. These behave optically as single crystals, but will not fuse into a single crystal during overgrowth.
of a single crystal-unit, and so all elements have identical
crystallographic orientation of c- and a-axes and overgrow as one unit,
e.g. apical spine of Sphenolithus heteromorphosus, entire nannoliths of Florisphaera, Marthasterites, Minylitha, Ceratolithus.