7. Crystallography

Calcite crystallographic orientation is precisely controlled during cocccolith growth and provides a key character for determining phylogenetic relationships. It also controls the appearance in light microscopy. Hence it should be described properly. This does require a modicum of knowledge of optical crystallography but nothing very tricky, good references are Young (1992b) and Moshkovitz and Ehrlich (1989).

7. 1 Crystallographic orientation

Calcite c-axis orientation can be, summarised with the following terms. Based on Young and Bown (1991), Young et al. (1992). N.B. Actual orientations can depart significantly (up to 30degrees) from true vertical and radial.
V-unit crystal-unit with sub-vertical orientation of c-axis. {Young and Bown 1991}
R-unit crystal-unit with sub-radial orientation of c-axis, relative to its point of origin (nucleation) on the proto-coccolith ring. {Young and Bown 1991}
T-unit crystal-unit with sub-tangential orientation of c-axis (e.g. Braarudosphaeraceae, Polycyclolithaceae). {Young et al. 1997}

conventions for showing crystal orientation

7.2 Graphical conventions for indicating crystallographic orientation

Symbols A single symbol per element can indicate c-axis direction.
Shading To directly illustrate observations made with a gypsum plate hatching can be used - vertical and horizontal for parts in extinction (purple). Diagonal for birefringent parts (blue and yellow). The direction of diagonal hatching should of course be based on the c-axis orientation and since the gypsum plate orientation varies between microscopes the relationship between observed colour (blue, yellow) and c-axis direction has to be determined for each microscope.
Unit type shading For illustrating structure it is convenient to apply the same shading to all the elements of one crystal-unit cycle in all views of the nannolith. For this the following scheme is recommended: V-units stippled; R-units blank; T-units dashes.

7.3 Light microscopy based terms

light microscopy terms
Birefringent/non-birefringent appearing 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 plateaccessory 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 fastc-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 slowc-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.
Monocrystalline formed 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.

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