Two pictures. At 550 and 1200x, you can really see the detail in the conglomerate mass of crystals. In the lower  picture, you can see thin cracks between the "boulders." I think those cracks are so small that an ordinary bacterial cell might have trouble squeezing inside. For reference purposes, a human red blood cell is about 7 microns in diameter, bacterium about 1 micron. A million microns will fit into a meter. Notice how when you really mag in, all sorts of caves and gaps appear in something which to the eye looks pretty smooth.