Talc is layers of octahedra sandwiched in tetrahedra!
Since these sandwiches form separate parallel sheets, I bet they can easily slide past each other. That must be why talc is so soft.
The octahedra are magnesium oxide and the tetrahedra are silicon oxide… with some hydroxyl groups attached to liven things up. The overall formula is Mg3Si4O10(OH)2. It’s called ‘hydrated magnesium silicate’.
The image here was created by Materialscientist and placed on Wikicommons under a Creative Commons Attribution-Share Alike 3.0 Unported license.