Tardigrades and polarized light (III)

In the scientific literature the tardigrade stylets are characterized as "partially calcitic" or as "containing calcite". As a consequence all tardigrades need calcium to survive. Maritime tardigrades like the ones from Losinj island (Croatia) which have been described in previous issues, will have no problem in this respect: the marine sand around the island is almost exclusively made up of calcite. When screening Losinj sea water sand samples for tardigrades we come across beautiful microscopic calcite crystals with typical calcite crystal habitus. These are ideally suited for microscopic studies of the calcite properties - and later on for the understanding of the respective physico-chemical properties of the tardigrade stylets.

Microscopic calcite crystal from a sand sample taken near Mali Losinj, on Losinj island, Croatia.

In order to make plausible that those crystals actually consist of calcite, we might pick them up and check whether they will react with acetic acid - normal vinegar will do the job. Calcite crystals should react by forming many gas bubbles (carbon dioxide) and vanish completely after some time. But of course we do not want to use this test on tardigrades whether they are alive or dead. Too brute, too primitive, a no-go!

So we will have to seek other properties of the stylets which might help to analyze the assumed calcite in a different manner - without murdering the tardigrades. Let's check it out: one further property of calcite crystals is their extremely strong double refraction behaviour which can be seen when studying these microscopic calcite crystals under the microscope. As the term "double refraction" already indicates, we will see double images (or ghost images): All scratches, inner irregularities, sharp edges etc. will appear as double images.

Tiny sea water sand grain crystal from Losinj island, Croatia, showing typical calcitic refraction behaviour (ghost images). Image width ca. 1 mm.

Even without a microscope we will encounter double refraction effects when studying bigger calcite crystals which are readily available via Ebay or from mineral dealers or trade fairs:

Larger calcite crystal showing typical double refraction

Calcite is among the champions of double refraction. Its double refraction (i.e. the maximum refractive index difference between the different light rays) has a value of 0.172. For comparison: The respective double refraction value of quartz is only 0.009. This means in practice that the distance between the ghost images is large for calcite and much smaller for quartz.

Double refractive material can also be recognized by x-y-turning the microscope slide between two polarizing filters mounted in "extinction" position or cross-position (cf. February magazine) where the specimens will appear periodically bright-dark-bright-dark in microscope specimen table rotation. But in the case of the tardigrades a simple proof of the stylet double diffraction property ("yes" or "no") alone wouldn't be very helpful: you see, many non-calcitic crystals are double refractive as well, so we would have to additionally prove the enormous strength of the double refractive character. That is where interference colours come in. Do you know why our big calcite crystal doesn't show any interference colours? The answer is: too big, too long interference pathways, a so-to-speak interference jungle ending up in shades of grey, no more distinct colours.

A small calcite crystal as seen under the microcope in polarized light. Please note that only the thinnest edges are showing interference colours. Image width ca. 2.5 mm.

Only very thin calcite crystals are showing interference colours under polarized light. You will be able to easily check for yourself, just scratch some calcite flakes from the walls of an electric water boiler and look at them under a microscope in polarized light:

Tiny, thin layer calcite crystals, from the walls of an electric water boiler. Polarized light, nice interference colours! Image width ca. 0.3 mm.

Sometimes you will come across crystals with wedge shape. In those cases you might be able to see that the interference colours come in a sequence which is dependent on the thickness of the respective calcite crystal layer. The interference colours of the thinnest regions appear in clean rainbow colours whereas thicker layers are showing more complex pastel colours.

Small calcite crystal thinning out to the right in a wedge type manner. Normal brightfield illumination, little to no interference colours. Image width ca. 0.3 mm.

The same crystal as seen under polarized light (thin edge on the right side). The colours become softer and more complex (pastel colours) when moving from right to left. Note that the "thinnest" interference colour is yellow (following the white edge region). Image width ca. 0.3 mm.

Now, what is the message in this bag of mixed and definitely confusing information? Calcite crystal interference colours will appear only in rather thin crystals of an "intermediate" thickness: the interference colours will not show up when the crystal is extremely thin (less than a few microns) or very thick. Due to its strong double refraction only calcite will start to show first interference effects in relatively thin layers where other crystals of the same thickness will show no colours at all. So, as the tardigrade stylets are only a few microns in thickness, any interference colour would clearly hint to calcite.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

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