If I use the shelf tool “source particle emitter” on a selected Houdini shape such as a grid or torus, the particles generated will be displayed as points in the Scene View. If I use the same tool on a group of faces selected from a model, the default display of particles seems to automatically display large black squares representing sprites. Is there a way to change this rendering to render particles as with the grid or torus shapes ?

Remove any shop_materialpath attribute before you plug your source into pop network.
This removes sprites and display particle as points.

I put an attribdelete node in the source geometry tree (to delete the shop_materialpath attribute (which had a /mat/blinn material on it and it works. I guess I don't understand why source objects can't have materials on them if you don't want to use particle sprites, but maybe I don't fully understand the particle sprite/rendering mechanism in houdini yet.

You can also disable sprite display in the Geometry tab of the viewport display options.

Thanks.. good to know..