Obviously it would be both cumbersome and confusing to have to draw ball-and-spoke models for the atomic structure of each individual tetrahedra.  Instead, mineralogists simplify the picture with a geometric symbol...

First, they simplify the picture by viewing the tetrahedra from directly overhead. (No more problems with drafting 3D perspective!)

Now what you would see is a pyramidal stack of 4 balls (oxygen ions). The tiny silicon ion is, of course, hidden in the middle.

Next, imagine a triangular pyramid draped over the ball model.

Now  get rid of the balls altogether. What you have finally representing the silicon tetrahedron is the geometric form of a  triangular pyramid (tetrahedron)

Each apex of the tetrahedron represents an oxygen ion. Again the silicon ion would be hidden inside.