What does it look like?
What is it?
Vermiculite is a layered silicate (also known as a phyllosilicates) mineral. It is very similar to Kaolinite in that respect by differs in the fact that it has an extra silicate layer (the blue tetrahedrons) that sandwiches the layer of magnesium (in this case) and oxygen (which are you can see as orange tetrahedrons). These layers themselves then sandwich other elements (often water molecules with some small metals) that can sit in-between. This layered structure means that the structure of a vermiculite and swell up to take up elements from its surroundings, but can shrink at high temperatures when the water is expelled from the sandwich.
Where did the structure come from?
There’s a lot of variation in the elements found in the vermiculite structure. This particular example is magnesium vermiculite, and is #9000146 in the open crystallography database.