Achromatium oxaliferum is the only one known bacterium in which
calcium (Ca++) is deposited in calcite crystals (crystallized
calcium carbonate). No one knows why.
You can see what this special microbe looks like in this nice
Achromatium oxaliferum by Michel-Marie Solito de Solis
The Vikings used this material to navigate their ships at sea! In
this context the mineral is called Iceland spar , and as a navigation
instrument it's called a Sunstone.
You can watch an explanation in this interesting video:
The Sunstone: Secret of Viking Navigation by Terra Incognita
maybe if the crystals in the creature are transparent, but we see
the creature as opaque because we see the light entering it on one
side and exiting it on another and the light passes two polarizers
(crystals) which are not in the same angle, so very little light
passes (like two filters).
But to the "eye" of the creature - if it has something like that in
its center - each ray of light goes through one polarizer (crystal)
only. So the creature can it can see polarized light.
Now the question what is it good for, you can read in Wikipedia in "polarization
and vision", which says it's good for navigation.
The creature lives in mud, which means normal visibility is not
good. but the mud is made of earth which means minerals, which means
crystals. light that goes through crystals become polarized, so this
gives the creature a view which is similar to radar!
plus maybe it can identify what's good to eat (identify certain
materials which are his food) this way.