There is no real set of elements defined for XAML. Instead, XAML is just a set of rules that govern the syntax of XML representing object graphs. The elements are simply class names, the attributes are just properties, etc.
Therefore if you're looking to understand what the XAML representation of a 3D model would be you really want to find the class that you would use and learn it's object model. I would start here.
HTH,
Drew