Apatite is a calcium-phosphate mineral commonly found in bones. That makes it important to dentists and orthopedists, for example. It also makes it a mineral the body already knows how to deal with i.e. it is in some sense pre-approved by nature for medical application. Thus for example apatite nanoparticles in blood might be developed as a contrast agent for magnetic resonance imaging.
What are the dimensions of this nano-crystal? What atom type resides in the four tubes or tunnels through this structure? A variety of atom-types (e.g. F, Cl, OH) fill these tubes in different types of apatite. What are the dimensions and angles of the unit cell for this particle (shown in blue)? What is the stoichiometric composition of the unit cell? How many different types of Calcium site do you find in this structure? What types of atoms sit on the very surface of the particle?
Below find a reduced resolution inset from a much larger transmission electron microscope image of apatite particles used as filler in cake mix.
The closeup below shows shows some curved 3.4A graphitic fringes in the carbon support film (lower left), as well as a closer look at fringes in one of the apatite crystals above. If this indeed is apatite, can you orient the lattice above so that it will match the orientation of the crystal seen below?